Need help with my story!?

Hey people am writing a story and its about a world war set in the future, i want to know in my story there invading countrys which include my country uk, the usa, russia, italy and germany, now i am not realy into politics and strategic things so i am unsure about this sort of stuff, so like if i am to invade countrys in my story what citys do i need to gain control of so my story makes sense, cause i dont want people saying well how can they invade that city thats a stupid thing to do or its impossible to attack there without wiped out etc etc if this makes any sense? Does it matter where the seize begins? what cities? army barracks? Thanks for reading