Erm even if the EU did unite into a single federal entity like the US, it still would not magically gain such power. It would still be only the second largest economy, soon falling to third place, third in population. It could be in theory powerful (although unlikely given the reluctance towards military spending in Europe along with a general lack of interest in exercising unilateral power amongst the populace along with many in Europe being sympathetic towards Russia) but it would not be THAT powerful.
Not powerful enough to dismantle Russia completely for example.
How exactly would that even be accomplished anyway?