How was Senator McCain tested when he responded to a question about a test in the first 6 months as president?

Joe Biden said last week that Senator Obama would be tried within the first six months of taking office. Clearly it's a reference to an attack of some sort on the USA or its allies.; maybe it involves terrorists or hostile nations. We don't know. Obama aggrandized it by stating that either candidate, McCain or himself, would probably be tested as President. McCain says he has already been tested. Besides his 5 years as a POW in Vietnam, how has he been tested on a presidential scale? Not a partisan question, just very curious.