It started when Franz Ferdinand died. Croata, Bosnia and Slovenia declare their independence and form The Kingdom of Serbs, Croats and Slovenes. Austria didn’t like this so they went to the new union to make them “reconsider their independence,” starting WW1. Germany needed another to maintain the balance of power so they got Spain to side with the Central Powers. 

Meanwhile, Austria-Hungary and Yugoslavia devolved into a civil war battle royale between all the different groups. Since France was busy fighting Spain, Germany focused on Russia, so Belgium never got invaded. Lenin was put in as a puppet when St. Petersburg fell. Germany then turned their attention to Italy. Going through a small chunk of what used to be Austria they marched onto Italian soil.

Since the war cut off trade, there was a mass famine going on. So someone could exploit this crisis to take power. That person was Leon Trotsky, since he wanted to spread Communism to other countries. Like how the USSR did during the Cold War. Basically, he did what Lenin did in our timeline.

Germany pulled out of WW1 making the war just Romania, UK, France,  Portugal and Greece vs Bulgaria, Ottoman Empire (Turkey),  Spain and Ireland. Alsi the ethnic battle royale in Yugoslavia and Austria. Trench warfare was still used everywhere except in that ethnic royale I just mentioned. The Centrals Powers eventually won.