As adversaries go, Benjamin Netanyahu and Mahmoud Ahmadinejad are oddly well-suited. The hardline Israeli prime minister and the fiery Iranian president seem to feed each other rhetorical ammunition to whip up fears that bolster them in domestic politics and beyond. Between them, they are stubbornly testing the limits of U.S. power in the Middle East and undermining the "new beginning" in relations between America and Muslims that President Barack Obama proposed in an eloquent Cairo speech nine months ago. Netanyahu contends that Iran is seeking a nuclear bomb to fulfill Ahmadinejad's declared wish for Israel's destruction. Confronting it, he argues,...