The U.S. rarely attacks or occupies those that didn't recently attack someone.
I mean, the most recent war with Iraq was something of a stretch, though there was at least some justification because of Iraq not holding up its end of international agreements regarding their nuclear program, which were made at least in part in response to Iraq attacking Kuwait just over a decade earlier. Otherwise, it's pretty much peacekeeping actions in response to U.N. decisions, helping out neighboring countries whose leaders ask, etc.
And when we say "keeps alternate parties off ballots", we don't mean "keeps alternate parties off ballots unless they can get at least n% of eligible voters to sign a petition. In the U.S., any party can get on the ballot, including those who just list one of the major party candidates as their candidate (yes, this is weird) just by getting enough signatures. If you can't get that many signatures, you were never going to win anyway, and all you can do is cause less desirable candidates to win by drawing votes from more desirable candidates.
Mind you, we've evolved into a two-party system that basically gives third parties little chance of actually winning any major election, which is, at least in part, a flaw in the way we do vote counting, but that doesn't prevent them from being on the ballot.