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Well the decision for it, does not have any prequisites, you can even be at war..
Technically, Russia WAS at war when the capital was moved. Although the game requires them to own the province which means it can't actually be done as it was historically.
 
Technically, Russia WAS at war when the capital was moved. Although the game requires them to own the province which means it can't actually be done as it was historically.

Well game is bugged anyway, the decision was supposed to need you to be western tech, and at peace, but for some reason, there is * before the requirement in game file, making it NOT work.
 
Denmark is the enemy war leader and HRE Austria is your ally. I gave it a quick spin on hard mode post-manpower changes and it doesn't seem too tricky although the couple of unfortified provinces and the endless Russians do present a fair challenge.

I'll try getting full occupation of all enemy provinces later if I can. Sweden is quite the powerhouse.
 
In the beginning of the war, Denmark usually starts by marching in to Skåne with like 20k or so. I usually starts by attacking them with 40k and stack wipe the Danish army. I would also seek an alliance with Holstein as they usually rival Denmark. Then I move over to the Baltic provinces and defeat the Russian armies one by one. The are at war with the Ottomans as well, so you won't have to deal with all of their forces. When their armies in Finland of the Baltic are defeated, they will retreat into Russia (if they don't get stack wiped) and then it usually takes some time before they come back. Use that time carefully. First, march south against the armies of the Commonwealth who usually starts bu besieging Riga with 35k. When the Polish army is defeated, go south and occupy Courland, but don't get stuck in Poland and Lithuania for too long, as by now the Danish have probably built up their forces and marched into Sweden from Norway. Dispatch 20k or so to go and deal with them. Meanwhile, try to avoid getting into combat with the Russians with your other army in the Baltic. When the Danish army is defeated for the second time, return to the Baltic, deal with eventual Russian troops and continue the War in Poland until you can get a separate peace with Saxony and annex Courland. I wouldn't recommend transporting your armies to the Swedish provinces in Pomerania to relieve them from the Saxon troops, since Denmark has naval superiority. When you have peace with Saxony and Poland, siege down the neighbouring Russian provinces (Pskov, Novgorod, Ladoga etc.), but don't adventure too far into Russia. When you have taken a couple of Rusiian forts bordering Ingria and Estonia, they will accept separate peace and give you Ladoga and some gold. By this point, Prussia often declares war. But before dealing with them, you should en the war with Denmark first, while they siege down you holdings in Germany. Their fleet is too weak to transoprt troops over to Skåne. Send all your troops and siege down Norway, kill off the remaining Danish armies and in the peace treaty you can force them to give you Trøndelag (which you have a core on) and if Holstein has sieged down Dithmarschen you can force Denmark to cede it to them. Force them to pay all of their ducats an war reparations. Finally if you want, you can force them to give you their colony in Africa which you have also a core on. Then send all your troops by boat to Holstein, combine with their army and defeat the much smaller Prussian army. If you want you can take their coastal provinces in the peace treaty. Don't care if the Austrians demand it, they usually don't attack you, and if so, get an alliance with France. but that's another matter.