Exactly, although the Dollar is potentially quite unstable given what the Chinese could have in store. Of course that being said any major economy collapsing would more than likely lead to an all out war, and war (due to the current political climate in western nations) would be suicidal for the U.S, and any other power involved due to the lack of stability most nations are experiencing. [spoiler] Simply put, waging a large scale war nowadays is impossible. Due to the rise of scepticism a draft would spark a revolution, and governments would struggle to maintain power over an already unruly citizenry, hence making armed conflict difficult to say the least. Not to mention all the political factions who might use such events to gain power.[/spoiler]

The US (as well as pretty much all nations) has historically thrived during war economies though. One of its biggest current problems is that a good portion of its populace doesn't support its current campaigns because they are not immediately at threat. Starting a war would not be in their interest but if someone else (say, North Korea) were to start an open war that immediately threatened the US (something that could rally the masses) it could be a boon to the US. China's collapse, however, would not work in their interest as it would likely lead to more of what we're seeing now, a conflict far away that the people want no part in. You'd have twice the problems (a huge blow to the economy and American intervention in a foreign crisis that the people would likely not support) with none of the benefits (war economy).

I highly doubt America could rally an army even if we were being invaded. America's political factions would see it as a chance to vie for power.