@Nari FFS Nazi Germany literally did just that before invading Poland, and the world did shit.
Annexed Austria, demanded a chunk of Czechoslovakia, marched troops into the rest of the country and said, "Ours now." Actually it was worst than that, UK and France broke up Czechoslovakia into chunks for several other European nations, and the remainder got invaded and occupied.
Fuck the most recent one was Russia and Crimea and EU slapped some sanction but let up cause Russia sells shit tons of natural gas to Europe, while Obama talked big but backed off and gave even Iran, Russia's ally in the ME some slack with that shitastic nuclear deal, by unfreezing Iranian government's overseas accounts and handing over $2billion. Outside of the Ukraine, how many people still care about Russia just snatching a port city connected to the Black Sea? How many people are still condemning Russia for that?
You on the other hand seem to be under some delusion that taking land during war and just "taking land" are somehow exclusive when they're not; the game of nations is always about land and resources. "Declaring" war is an after thought, a civility. Moving armed forces nearby another nation's border was enough to start wars in history, because it was a pretext for taking land!
And yes, I think the world would just let the US take over a Canadian city. Who's gonna have to step up to do something about it, without going nuclear? UK? France? Germany? They're gonna cut out a massive market, AND their own investments in the US, while cutting US investments into Europe with sanctions? The Middle East? Wtf do they care, they have their own problems with each other. India, Pakistan, and China are at each other's throats. Russia not gonna do anything either. The most would be some condemnation but every major power is so tied into each other through trade and investments, that cutting off the US through even sanctions, let alone military action, would spell doom for the entire world economy.
I mean, what did the fucking world do when the US just up and invaded Afghanistan and Iraq? FUCKING SHIT. Hundreds of thousands of dead Iraqis, Bush certainly could've been tried and found guilty of war crimes, but what happened? Words of condemnation and words are cheap.
Edit: I mean, fucking 20+ nations joined the curbstomp party on Iraq for that matter, and everyone involved had a great time shooting Iraqis until they realized that the whole justification for invading was shit and the US has no actual gameplan for occupying the nation WHICH MAKES IT EVEN FUCKING WORSE.
Now let's assume we go into a hot, non-nuclear war; for either EU or China/Russia, they'd have to send a fleet of ships, to support an invasion force, across several thousand miles of ocean, AND establish a supply chain for that army and fleet, WHILE battling one of the most advanced navies in modern times on the sea, along with FIGHTING one of the most powerful land based militaries in history, AND that's not accounting for any civilian "volunteers" that'd pop up if said invading army landed on the continental US.
