President Trump won applause in Tuesday night’s State of the Union address when he declared that “great nations do not fight endless wars.” It’s a resonant line in a country that has been fighting in parts of the Middle East for nearly two decades. And, in a literal sense, the statement is true.

Yet the risk of Mr. Trump’s rhetoric is that it encourages a public belief that America can abdicate its responsibility to work with allies to preserve the peace. History shows the great danger in failing to distinguish between fighting...