The implications of Donald Trump’s abrupt withdrawal of U.S. troops from Syria will play out over many months, but a portent was apparent Wednesday. While Mr. Trump was defending his Syria pullout during his visit with U.S. troops in Iraq, Russia was condemning Israel’s defensive air strikes in Syria. This is only some of what will fill the vacuum left by America’s departure.

Israel rarely acknowledges its military actions, but for months it has been hitting targets inside Syria to deter and degrade the buildup of Iranian...