Why is he always telling them what he’s not going to do? This has become a constant refrain among President Obama’s Republican and conservative critics. And it is an excellent question. Why does the president tell jihadists from the Islamic State and al-Qaeda that the American campaign against them will be strictly limited to aerial bombing (sporadic, at that) and absolutely, positively will not involve the introduction of U.S. ground forces? The theory behind the question is bulletproof: the only way you can hope to keep bad actors in check, to discourage them from acting roguishly in pursuit of their...