Hearts of Iron 4
This has been my most successful run at being a major power ever so far.
I picked Nazi Germany but then immediately went for Oppose Hitler in the focus tree, fought in a German Civil War, and now Hitler and the Nazies are dead in 1936. In rebuilding the nation you can go democratic or restore the German Empire, I've done democratic Germany before so I picked German Empire and Wilhelm II came out of exile. At the same time though Britain turned fascist (which I think is scripted because of the German civil war, something about "The British Union of Fascists" which was a real thing that was dissolved in 1940 in our timeline) which has made them enemy #...2, I deemed the Soviets the bigger threat.
I can't fight alone though so I formed the faction "Central Powers" and first invited Bulgaria, then the Austrians, Hungarians and Czechs voluntarily formed Austria-Hungary and joined next, then in 1939 I had Benito Mussolini assassinated and brought the Kingdom of Italy into the fold (and depriving fascist Britain of it's only ally at the time), then Romania asked to join too. I then guaranteed the independence of Lithuania, Latvia, Estonia and Finland, and then did the same for Poland in exchange for Danzig (and they agreed!). Later on Poland and the others also officially joined my faction as world tension was rising and the Soviets moved to make demands on Poland.
The Soviets attacked Poland and the German-Soviet War had begun as I called all of the faction to war. With close to 3 million casualties on our side and 4.90 million Soviet casualties, the war was already over in December 1941 with us the victors. I took no land myself to avoid too much border gore, I made Russia a puppet within the faction and neatly gave a nice chunks of land to Finland, Estonia, Latvia, Poland, Romania, and on the way home I got Italy a piece of Yugoslavia, because they were actually competent in this timeline, including successful naval landings in Crimea, the Caucasus, and northern Russia which was a surprise to be sure but a welcome one.
But in the west fascist Britain gained the Netherlands and France as allies, and now another war in on the horizon as my vast faction now marches west against the fascist threat. Because I was fighting an entirely land battle it means that my growing fleet was out of harms way, so I now have a whole bunch of battleships, the heavy fighters, naval bombers and submarines to support them, and I've even started building aircraft carriers, I plan on going toe to toe with the British navy as an obstacle to my invasion. Hitler's dead, Mussolini's dead, Stalin's gone, you're next random British guy not from the history books.
Plus I'll get to both liberate and unify Ireland, because you can bet an invasion of Ireland is the first thing fascist Britain would do...
Edit: Okay maybe not unify, because I think that would stop me from getting an achievement, maybe later?