While optimism is a laudable characteristic, it can be dangerous if not tempered by cold realism.
A second scenario for a U.S. defeat might be called “grab and dig in.” An adversary would launch a surprise attack, seize territory and then hope that the United States will decide that the cost of rolling back the invasion is too great. This is what Japan did at the beginning of World War II. It didn’t work then because Americans were incensed by the attack on Pearl Harbor and felt that they could reverse the Japanese gains at a reasonable cost. The island battles in the Pacific were horrible, yet tolerable for an angry nation.
But what if a future invader had nuclear weapons? Imagine territorial aggression by China, Russia, North Korea or a nuclear-armed Iran. Any of these potential adversaries would put their vital national interests at stake in a conflict, while the United States might not. A threat to use nuclear weapons by such hypothetical aggressors would be credible. American policymakers then would have to decide whether it was better to risk a nuclear exchange or simply accept a land grab such as a Chinese annexation of Taiwan, a North Korean occupation of part of South Korea, a Russian seizure of a Baltic state, an Iranian takeover of Shiite-majority territory in Iraq, or something similar. Put differently, if a nuclear-armed adversary avoided the mistake made by the Japanese in 1941 and did not directly attack the United States, chances are that American policymakers would grudgingly tolerate its aggression.