4. Do a Republican Serbia in Kaiserreich.
This was probably the longest game I've ever played, as usually I cut them off at a 10-year mark, but this went on past '47...
Because it was difficult.
First, Serbia managed to get the jump on Bulgaria, securing Macedonia in the first month of war, then slowly ground down the rest of Bulgaria until total occupation was achieved. This was done to prepare Bulgaria for full integration into a future South Slavic state. Next, Egypt was wrecking the Ottomans, presenting a fine opportunity to retake the city of the world's desire, Constantinople. Also, the Greeks had betrayed the Belgrade Pact, so no need to share the spoils! The Ottomans were easier to defeat than Bulgaria. Serbia reclaimed the imperial city, along with Thrace and Agean territories. Anatolia was set up as a puppet occupation regime, subject to future integration.
Next, the Geeks were punished for leaving the alliance to instead go gallivanting with the Entente, who were once again wasting their armies failing to invade Britain. Since Egypt was grateful for the assistance against the Turks, they gave us permission to transit across their territory, to invade French North Africa. Also, the Turks still had a navy, which helped Serbia seize various Mediterranean islands.
However, the main prize was the yet un-liberated slavic territories held by Austria. Austria was helping the Reichspakt, and while Germany stood, no one would help Serbia against this much larger foe. So Serbia decided to give the Internationale a boost to help break the stalemate, so that we would have help against this final enemy...
First, Serbia moved from North Africa along the Moroccan coast to take the rest of German Africa. From Tunis, Serbia invaded Sicily, and ended Italian participation in the war. In Spain, the Kingdom was giving France much trouble, so units made the crossing at Gibralter to take them out. In the East, Russia had taken over the Baltics, Scandinavia, and Ukraine. The last front was the Western Front, and all was proceeding smoothly... So the time had come to invade Austria.
The French and Russians had pushed right up to the German borders, leaving them alone with no allies to distract the war effort. It would be only a matter of time before German defenses were breached. In this downtime, Serbia prepared its invasion army. Heavy Infantry Support Tanks was distributed to all divisions, adding some armor that would help with the breakthrough, while also reducing losses. Already, the manpower to fill a field army was tight even with all adult men in service. When Turkey was integrated, a third of a million of their manpower was added to buff up the invasion forces.
Then, Romania betrayed the alliance to join Germany, so Serbia quickly occupied them before they could interfere with the plan.
Finally, the dice were cast. Although the tanks did protect the men from unnecessary damage, the Austrians knew we would come, and had dug in deeply at the border. It took long, painful, concentrated attacks to bite-and-hold territory. This went exactly as the grinding trench warfare of old. For although the Serbians had superior tactical advantage with their tanks and air superiority, the Austrians had much larger industry, army, and manpower pool... or did they?
The losses of the first offensive meant Serbia had scape the bottom of the barrel, severly hurting factory output due to the amount of workers dragged into the frontlines. However, out spies report that Austria remains on peace-time conscription and have no reserves left. Every loss we inflict, they will not replenish. And while they have more industry, guns without soldiers do not win wars. So, the leadership of Serbia makes the painful decision to press on the attack, grinding down the enemy in brutal attrition, because our loss rates are equal, but we have a larger reserve.
Mathematically, victory is certain... the balance is clearly in our favor... at least... until the Japanese attacked.
The Japanese had been quietly conquering all of Asia, sniping away at vulnerable colonies, manipulating Chinese warlords. They even got in good grace with MacArthur by giving him back Hawaii, while having the same ideological outlook of control.
Now, the Japanese were suppressing syndicalist movements in Asia, which brought them into conflict with the Internationale, so they invaded Siberia, and used Portugal as a stepping stone to invade Europe. Spain fell, and the French borders were breached from the south. The Germans launched a desperate last offensive, which took France to its knees. Paris fell, the Russians lost Ukraine, allowing German units to pour through Romanian plains and threaten home soil! all hope was fading...
Then, suddenly, deliverance came when the Austrian lines collapsed. Having started the war with no reserves, the mounting losses meant that their army was disintegrating. Slowly at first, with the fall of Budapest, but then faster and faster, as weaker units gave more ground and lost more supply and became even weaker, putting them in a downward spiral. Vienna, Venice, and Stanislav fell. Serbian units rushed north to rescue Russia, passing along the Polish border to Lithuania, trapping large German armies in Ukraine. Although the Germans all but knocked France out of the war, with the Austrian surrender, Allied troops invaded Germany from their long undefended southern/eastern border. Germany surrendered, and the war was over.
Serbia has united not just all Southern Slavs, but the entire Balkans. Although many call for the new power to be named Yugoslavia, a proposed alternative could be the Republic of Byzantion.