The Wall street Journal, June 19, 2017 – A bipartisan conceit has been that the U.S. can defeat Islamic State by ducking the larger conflict in Syria, and now we’re finding out that may not be possible. A U.S. F-18 jet shot down a Syrian bomber on Sunday to protect U.S. allies fighting Islamic State, and on Monday Russia and Iran threatened to target U.S. planes in response.

A U.S. fighter shot down the Syrian SU-22 plane after Syrian aircraft made their second bombing run against Syrian Democratic Forces (SDF) allied with the U.S. near Taqba. The regime was clearly testing whether the U.S. would assist its allies on the ground. The U.S. needed to send a deterrent message or Syrian President Bashar Assad will continue to press his offensive across SDF-held territory.

The risk of escalation is real, but this isn’t a skirmish the U.S. can easily avoid. Mr. Assad and his allies in Moscow and Tehran know that ISIS’s days controlling Raqqa in Syria are numbered. They want to assert control over as much territory as possible in the interim, and that means crushing the SDF.

The Russian threat on Monday to target with anti-aircraft missiles any U.S. aircraft flying west of the Euphrates River in Syria is part of the same intimidation strategy. Russia also suspended a hotline between the two armed forces designed to reduce the risk of a military mistake. Iran, which arms and assists Mr. Assad on the ground, vowed further Syrian regime attacks against SDF, all but daring U.S. planes to respond amid the Russian threat.

The White House and Pentagon reacted with restraint on Monday, calling for a de-escalation and open lines of communication. But if Syria and its allies are determined to escalate, the U.S. will either have to back down or prepare a more concerted effort to protect its allies and now U.S. aircraft.

This is the predicament President Obama put the U.S. in when his Syrian abdication created an opening for Vladimir Putin to intervene. Had the U.S. established a no-fly or other safe zone to protect refugees, the Kremlin might have been more cautious. Mr. Putin took the measure of Mr. Obama and gambled the former U.S. President would protest and do nothing. He was right. Now the Russian is testing President Trump as everyone maneuvers for post-ISIS advantage.

As a candidate, Mr. Trump supported “safe zones” for refugees and opposition forces. But he’s also shown no interest in a larger strategic goal than defeating ISIS. Now is the time for thinking through such a strategy because Syria, Russia and Iran know what they want.

Mr. Assad wants to reassert control over all of Syria, not a country divided into Alawite, Sunni and Kurdish parts. Iran wants a Shiite arc of influence from Tehran to Beirut. Mr. Putin will settle for a Mediterranean port and a demonstration that Russia can be trusted to stand by its allies, while America is unreliable. None of this is in the U.S. national interest.

The alternative would be to demonstrate that Mr. Assad, Iran and Russia will pay a higher price for their ambitions. This means refusing to back down from defending U.S. allies on the ground and responding if Russian aircraft or missiles attempt to take down U.S. planes. Our guess is that Russia doesn’t want a military engagement with the U.S. any more than the U.S. wants one with Russia, but Russia will keep pressing for advantage unless President Trump shows more firmness than his predecessor.