On September 14, 1939, having concluded the Polish Campaign, the main German forces began withdrawing from Poland back to Germany. The three French armies that had been attacking the Siegfried Line were ordered to retreat.

The French Ministry of Defense did not publicize this offensive campaign, even avoiding mention of it as much as possible. This was not because the tens of thousands of French troops didn't fight hard; during the month of combat, they not only conducted standard infantry assaults but also utilized aerial bombardment and tank assaults, firing hundreds of thousands of artillery shells.

However, a modern defense line is not so easily breached. The French army exhausted every tactic they had mastered, including the extensive use of the German Stormtrooper infiltration tactics from World War I, employing large numbers of elite small units for infiltration attacks along the extensive front. Ultimately, with over ten thousand dead and tens of thousands wounded, the French Ministry of Defense, seeing that the offensive was unsustainable, had to order a retreat.

In response, while reviewing the lessons learned from attacking fortified positions, the French Ministry of Defense also tried to gather intelligence to understand how the Germans achieved victory. Marshal Gamelin of France proposed a suggestion to the British commander in the coalition: since attacking modern defense lines was ineffective, it would be best to form an army group consisting of French, British, and Belgian troops to launch an offensive against the German mainland from Belgium.

Faced with this request, both Britain and Belgium voiced their opposition. Belgium, in particular, was resolutely against it. In the eyes of the Belgians, the French harbored ill intentions. In 1930, when France built the Maginot Line, they asked Belgium to build defenses as well, but the line was situated to the south of Belgium. Once war broke out, the French could hide behind their fortifications while Belgium would have to stand in front and fight to the death against the Germans. Thus, Belgium stated at the time that if the French did this, Belgium would consider France a hostile entity.

Now, Britain and France had declared war on Germany without consulting Belgium, and were trying to force Belgium to join France in an offensive against Germany. Belgium could find no justifiable reason for this. Moreover, pro-German sentiment was strong within Belgium, especially with King Leopold III, who was completely unwilling to side with France against Germany.

Unable to organize an offensive against Germany, the French government had to choose a passive defense. They launched large-scale conscription domestically, and military industrial enterprises ran at full capacity producing various weapons and equipment. By January 1940, the employment rate of French women had reached an unprecedented 51%, and women in worker uniforms undergoing training could be seen everywhere in arsenals.

At the port of Marseille, a fleet of 12 ocean liners flying the Chinese flag docked on January 14, 1940. A large number of French youths disembarked and immediately reported to recruitment centers. In recent years, Sino-French trade had been very active, with increasing personnel exchanges between the two countries. There were nearly 100,000 French people in China, more than half of whom were young people and technicians. Upon receiving their draft notices in China, this group immediately chose to return to serve their country, taking advantage of the Chinese evacuation fleet to return to France.

Once the French had disembarked, Chinese merchants, students, and expatriates who had been waiting at the port of Marseille immediately boarded the ships. The European war was an unexpected disaster for the Chinese in France. Under the threat of war, most Chinese in France chose to return home.

In the past half-month, the Chinese Embassy and consulates in France received nearly a thousand requests for certification of Sino-French marriages. Many Chinese had come to France alone. Some had married French wives or had French fiancées. Before the war broke out, these Chinese had hoped to live peacefully in France for the long term. With the outbreak of war, everyone felt that China was safer and prepared to return home to wait out the war.

According to Chinese tradition, one could not leave behind one's wife or fiancée, so they naturally had to be taken along. Although French fathers-in-law might not have been willing to see their daughters go to China, thousands of miles away, surprisingly few opposed it.

Consequently, French churches came under immense pressure from this sudden wave of transnational marriage applications. If France weren't a "revolutionary base area" of Europe with a high tolerance for marriages where one party was not Christian, many French girls might not have received the priest's blessing.

Arriving with the fleet were three Chinese Air Force groups. Of the five escort air groups the Chinese government planned to dispatch, three were army aviation groups, and the other two were carrier-based fighter groups from the Chinese aircraft carrier formation. The two aircraft carriers, already launched, were undergoing outfitting around the clock; their earliest deployment would be late February 1940. Due to their draft and width, Chinese carriers could not pass through the Suez or Panama Canals and had to go around the Cape of Good Hope in Africa to enter the South Atlantic, then sail north to the North Atlantic. The journey would take two months, arriving near France in late April at the earliest.

Nevertheless, the French government warmly welcomed the Chinese aviation units. They arranged for the land-based Chinese aviation forces to be stationed at airbases centered around Marseille.

This news not only attracted the attention of many French newspapers but also aroused the curiosity of the French public. Early in World War I, when Russia was still a member of the Entente, a rumor circulated in France that hundreds of thousands of Russian troops would arrive. The French public believed it true and went to train stations to welcome the Russian allies, waving Russian flags whenever a military train appeared. In the end, they waited until the war ended without ever seeing the Russian allies.

Initially, French newspapers were skeptical that China had actually sent planes. It wasn't until reporters saw Chinese pilots and ground crew at the airbase, flying French and Chinese Air Force flags, and interviewed the Chinese Air Force, that the news shook France.

Even though French newspapers clearly stated that "The Chinese Air Force is here to escort the evacuation fleet," the French public naturally ignored this. The French people believed that the Chinese Air Force had come to join the war under the guise of escort duties; France's Chinese ally had finally arrived!

The Nazi German government also received this news, and their view was similar to that of the French public. They believed the Chinese Air Force's presence in France was not merely a formality and that aerial combat with Germany was inevitable. However, Nazi Germany's intelligence network went into full operation and confirmed that China had indeed sent only about 300 fighter jets, so they said nothing further.

France currently had only 900 modern aircraft, and with Britain's 500, the total was 1,400. To attack France, the German military had prepared nearly 4,000 fighter aircraft. For the Anglo-French coalition, 300 Chinese planes were very important. For the German military, 300 Chinese fighters would be an opponent, but one at a complete disadvantage.

On January 27, a standard German civilian car appeared in the direction of the Ardennes Forest. Although it was wartime, civilians were still allowed to cross the border. Belgian soldiers at the border checkpoint checked their documents and looked through the open window. In the back seat sat two men in civilian clothes, one thin, the other sturdy but also lean.

The car had been checked and carried no weapons, so the Belgian soldiers let it pass. Before the vehicle had gone far, an officer walked out and asked who had passed. The soldiers hurriedly saluted, "Report, sir, two German engineers."

"Engineers?" The officer wasn't very surprised. German engineers were skilled and affordable; many Belgian companies hired or had hired them.

A soldier replied, "Yes. I smelled a strong scent of engine oil on one of them. He had bathed, but the smell was still there."

The Belgian officer was reassured. In his view, neither spies nor German generals would spend all day around machinery, and German generals should be in their headquarters right now—how could they run to Belgium in the dead of winter?

Contrary to the Belgian officer's guess, the two men in the car were Manstein and Guderian. After passing the border checkpoint, the car drove along the road in the Ardennes Forest towards Belgium at a moderate pace. Looking at the terrain of the Ardennes through the window, after a while, the car stopped, and the two relieved themselves by the roadside. Afterward, Guderian walked to the middle of the road, pounded the surface with his fist, and pried at it, dislodging some paving material.

Manstein jumped off the road to check the thickness of the roadbed. Then the two got back in the car and continued. Along the way, they stopped frequently to examine the terrain and the roads. Both agreed that the Belgian roads were well-built, but neither believed that these few roads through the Ardennes alone could provide sufficient transport for Manstein's plan.

Both were military strategists. After entering the border between Belgium and France, they changed routes and turned back. constantly examining the terrain and combining it with the current cross-country capabilities of German armored units. Once back in Germany, they immediately took a train back to Berlin.

Upon returning to Berlin, they immediately went to see General Rundstedt. During the Polish Campaign, Rundstedt was the commander of Army Group South, and Manstein was his Chief of Staff. Guderian had led the spearhead of Army Group North.

Learning that his former Chief of Staff and the fierce general from Army Group North had arrived, Rundstedt immediately invited them in. Manstein and Guderian saluted the General, then handed their respective reports written on the train to Rundstedt. However, the General did not look at them; he gestured for the two younger juniors to sit down and asked, "What is your opinion?"

The two answered almost in unison, "It is credible for large-scale armored units to pass through the Ardennes Forest!"

Manstein and Guderian hadn't gone on a trip to Belgium out of boredom. After Germany and the Soviet Union partitioned Poland, they immediately prepared for war with France. The initial plan devised by the General Staff was basically the same as the Schlieffen Plan used by the German army in World War I: bypass the Low Countries of the Netherlands and Belgium and attack France along the coastline.

There was nothing inherently wrong with this plan, but France had now built the Maginot Line. Judging by the French army's experience attacking the Siegfried Line, a frontal German assault on the Maginot Line would be protracted and costly. Since the German army certainly couldn't do that, the remaining direction of attack was the Low Countries.

Manstein certainly understood the General Staff's thinking, but he was furious. At a General Staff meeting, he openly declared, "It is a disgrace for us to use an operational plan from the last era in this one!"

The German General Staff didn't dislike sharp elite officers like Manstein, so they asked for his plan. Manstein proposed attracting the main force of the Anglo-French coalition to the Netherlands and Belgium, while German armored forces would pass through the Ardennes Forest in Belgium, bypass the Maginot Line to enter France, capture Sedan, drive all the way west to the English Channel, cut off the connection between the main Anglo-French forces in the Low Countries and the French rear, and defeat France in one stroke.

Once proposed, this plan immediately received support from a few high-ranking German generals, including Rundstedt. The German General Staff conducted a detailed simulation and assessment of Manstein's plan and ultimately concluded that the risk was too great; the main German armored force could not achieve the critical step of crossing the Ardennes Forest.

The German General Staff was well aware of the high-quality roads in the Ardennes; during World War I, they had even conducted detailed mapping of the area. Based on the performance of German armored forces in the Polish Campaign and assessments of logistical requirements, sending at least 2,000 tanks across over a hundred kilometers of the Ardennes region was too risky! Compared to risking Germany's most elite and precious armored forces, letting them gallop across the vast plains of the Low Countries was the most rational choice.

A major reason Manstein proposed this plan was that he personally couldn't accept the General Staff's plan. Since the General Staff deemed it unfeasible, Manstein sought help from tank expert Guderian. Guderian had combat experience in the Ardennes during World War I. Based on his memories and understanding of armored warfare, Guderian told Manstein, "I think it works. But you have to go there with me again." This led to their personal trip. After seeing and inspecting it with their own eyes, they were even more firm in their judgment.

Seeing that the two had reached a consensus, Rundstedt told them, "I will find an opportunity to propose this plan again," and sent them away.

The next day, Manstein received transfer orders from the General Staff. He was promoted to Lieutenant General and appointed as an infantry corps commander in Poland. Manstein wasn't surprised by this, merely snorting coldly in his heart.

Manstein's biological father was a German general, and his aunt married Lieutenant General Manstein. The couple had no children, so shortly after his mother became pregnant with him, the two families decided to have the unborn child adopted by his uncle-in-law. Another of Manstein's aunts married former President Field Marshal Hindenburg. The military's trick of promoting in rank but demoting in power to kick someone out held no secrets for Manstein.

Since the old fogeys at the General Staff were so hidebound, Manstein decided they couldn't blame him for playing dirty!

According to the rules, operational plans formulated by generals had to be approved by the General Staff before being presented to the Head of State. Bypassing the chain of command was seen as great contempt for the General Staff and a violation of the rules. However, the fact that this was considered breaking the rules meant someone had done it before. In the era of the Second Reich, Manstein's plan could have been submitted to Kaiser Wilhelm II through his uncle Hindenburg. Although Hindenburg was gone, Manstein was not without opportunities to submit the battle plan to the current German Führer, Hitler. Of course, regardless of whether Hitler approved the plan, Manstein's actions were destined to incur the extreme displeasure of the General Staff!

Manstein had decided that even if it meant breaking the rules, he would do it. For someone of his background, finding an opportunity within the rules was a very simple matter—after all, the rules were set by his ancestors' circle. The opportunity came quickly. In January, a German military plane got lost on the Belgian border and made a forced landing in Belgium. The officer on board carried the full set of operational plans for the invasion of France. Therefore, the German General Staff believed the plan had been compromised.

In fact, the General Staff's thinking was wishful. There were many anti-Hitler and anti-war officers in the German military now. Most of the contents of Case Yellow had already been leaked to Britain and France, and the Anglo-French coalition had conducted numerous simulations themselves, arriving at a German operational plan not much different from Case Yellow. What Britain and France really cared about was the specific deployment of German troops and the timing of the attacks by various units.

At this time, the Chief of Staff of Manstein's corps had to return to Berlin to report to Hitler. The Chief of Staff himself was a member of the "Young Turks"; he had served under Manstein during the Polish Campaign and was won over by Manstein's brilliance. Moreover, he also disagreed with the General Staff's plan. Since Manstein wasn't afraid, the Chief of Staff was willing to risk it all too. in his view, if they followed the General Staff's plan, their infantry corps would sooner or later be transferred to France to fill trenches. Rather than fighting into a sea of corpses without guaranteed victory, it was better to gamble now and find a sure-fire operational plan for Germany.

On February 4, while reporting to Hitler, the corps Chief of Staff privately submitted Manstein's plan to him.

Hitler was already very dissatisfied with the General Staff's plan. According to it, the German army was bound to get bogged down in a bitter struggle. And behind Germany was the completely unreliable Soviet Union. After the Polish Campaign, Hitler ordered Ribbentrop to go to Moscow again, hoping the Soviet Union would join the German-led alliance. Unlike the last time, Stalin not only refused but also demanded that Bulgaria become a Soviet sphere of influence before the Soviet Union would consider continuing negotiations on an alliance with Germany.

This action alone convinced Hitler that Stalin was unreliable. Although Ribbentrop used the model of mutual security and trust between China and the Soviet Union to persuade Hitler, arguing that establishing a military buffer zone could stabilize Stalin, this time Hitler completely rejected Ribbentrop's suggestion. He even asked, "If the ruler of the Soviet Union were He Rui, I would be very willing to establish a military buffer zone with him. Because He Rui is a person who understands what peace is, and has the ability and will to maintain peace. Ribbentrop, do you think Stalin understands peace?"

Ribbentrop pondered for a while before answering, "Mein Führer, I think Stalin is a very standard Russian."

Hitler did not discuss the matter further. Even if he saw through Stalin, it wouldn't solve Germany's current situation. Without an operational plan capable of defeating France, Germany's rear would always be in danger.

So, when faced with an operational plan obtained through unofficial channels, Hitler was not only not displeased but actually felt some anticipation. Daring to submit a plan of this level through unofficial channels meant the other party must have some unique ideas!

With anticipation, Hitler ended the meeting and immediately opened the plan.