I'm just finishing Anthony Beevor's history of the Second World War. His book about the Battle of Stalingrad, btw, is magisterial.
Anyway, this passage struck me as germane to the topic:
"There is, nevertheless, a real danger of the Second World War becoming an instant reference point, both for modern history and for all contemporary conflicts. In a crisis, journalists and politicians alike instinctively reach for parallels with the Second World War, either to dramatize the gravity of the situation, or in an attempt to sound Rooseveltian or Churchillian. To compare 9/11 to Pearl Harbor, or to liken Nasser and Saddam Hussein to Hitler, is not just to make an inaccurate historical parallel. Such comparisons are gravely misleading and risk producing the wrong strategic responses. Leaders of democracies can become prisoners of their own rhetoric, just like dictators."
Going back upthread a bit, addressing kcgrew—it was more than just domestic isolationists that kept the US out of the war when Hitler was romping around Europe from '38-'40. The Western democracies simply didn't have a military force that could stand up to the Wehrmacht in those years. No amount of tough talk or bellicosity or pointless sacrifice would have changed that. Had Hitler not hastened the inevitable clash of the two totalitarian states, it might have been many years before the western democracies could have rolled back the Germans' mastery of Europe. As it was, doing so meant making a deal with the devil and ultimately consigning miillions of Eastern Europeans to the depredations of the USSR. It's harder for democracies to move to a war footing, but it is well that it should be so.