Last week, despite Donald Trump’s repeated pledge to end American involvement in the Middle East’s conflicts, the U.S. was on the brink of another war in the region, this time with Iran. If Iran’s retaliation for the Trump administration’s targeted killing of Tehran’s top commander, Maj. Gen. Qassem Soleimani, had resulted in the deaths of more Americans, Washington was, as Mr. Trump tweeted, “locked and loaded” for all-out confrontation.

Why does the Middle East always seem to suck the U.S. back in? What is it about this...