The British had landed on the French coast and quickly captured Brittany and Normandy. Their army planned to seize control of France and then move to Spain.
This plan failed because of the other two allies. From Alsace, the Germans moved into eastern France, and, from the Low Countries, the Russians moved toward Burgundy. It seemed clear that France was lost to the Marxists, but the UMRE decided to make a stand there anyway - France had been the first of the Western European nations to fall to the revolution, and the new union was clearly worried that the UMRE couldn’t survive its fall.
As it happened, they were correct, but their hyperfocus on the French Front meant that they were unable to defend other portions of their territory. In the middle of 1941, the United States finally moved against the UMRE, landing soldiers in Andalusia, Sicily, and Sardinia. They subsequently joined with the British Navy to clear Marxist forces from the Mediterranean, which increased the importance of France. Now, should France fall, the UMRE would be divided into two states - Spain and Italy - surrounded by enemy territory.
The Marxist army fought a last stand on the Rhone River. It was a desperate move - a loss would annihilate the last military forces that the UMRE had because the Westerners threw everything they had into the battle. They assembled any ship - no matter how old - into a slapdash navy and used that to seize control of the Rhone River itself. They joined their few remaining armies, which were defending against the advances of the three allies, together and even raised militias to defend France.
France was extremely strategically important before these moves, but a victory wasn’t vital. After the Marxists stripped all of their military assets to attempt to eke out a victory, a loss on the Rhone River would destroy the UMRE completely, removing all of its legitimacy and stripping it of all of its territory. Both sides knew this, and both fought as if it were true.
At first, the gamble appeared to be paying off for the UMRE - their ramshackle force pushed the British back to Normandy, kicking them out of Brittany and the occupied lands in France proper. They held the Germans back from Burgundy, and the Russian advance was stalled. For one glorious moment to the Marxists, it must have seemed like the loss at Brussels would prove irrelevant.
Yegorov the Younger rose to the challenge. He arranged a meeting with both Britain and Germany to propose a unified operation in France. This meeting, which took place at Amsterdam, also created preliminary occupation zones for Western Europe, although that agreement didn’t have official approval.
The plan was simple - the Germans and the Russians would unite in Burgundy and advance, hoping that their superior numbers would allow them to reach Normandy and unite with the British. The united army would employ scorched earth tactics to devastate the Marxist defenses and break the will of the UMRE’s citizens. Meanwhile, the British navy would unite with the Russo-Swedish one in the English Channel and subsequently move first into the Atlantic and then into the Mediterranean, from which they would enter the Rhone and destroy the French navy. With their naval control of Southern France assured, the only resistance would be in the north - and that could easily be crushed.
This plan worked, but it took a while and had to be modified to keep up with the operations of the Americans and the Spanish. France was fully occupied by May 1942, but the United States had already occupied most of Italy by that point, and most of Spain was already liberated. This meant that the cleanup operations on the part of the three powers was simple - occupy the northern parts of Italy and Spain.
The Marxists were completely crushed by 1943, but the war was costly. The victors lost many men, but the UMRE was devastated - while the Low Countries got off relatively unharmed, France had large tracts that no one could live in due to lack of food, and Spain and Italy had lost much of their population. The Rhone Campaign was not extremely long, but it irreparably harmed Western Europe.
The victorious allies were left with thousands of miles of territory to occupy, a wasteland to reclaim and organize, and an idea to utterly destroy. They met in Nantes, the only French city of any historical significance left unharmed to determine the post-war order.
On his way to the meeting city, Tsar Nicholas was said to weep over the destruction.