Sunday, a Navy F-18 Hornet shot down a Syrian air force jet, an act of war against a nation with which Congress has never declared or authorized a war.

Washington says the Syrian plane was bombing U.S.-backed rebels. Damascus says its plane was attacking ISIS.

Vladimir Putin’s defense ministry was direct and blunt:

“Repeated combat actions by U.S. aviation under the cover of counterterrorism against lawful armed forces of a country that is a member of the U.N. are a massive violation of international law and de facto a military aggression against the Syrian Arab Republic.”

An ABC report appears to back up Moscow’s claims:

“Over the last four weeks, the U.S. has conducted three air strikes on pro-regime forces backed by Iran that have moved into a deconfliction zone around the town of Tanf in southwestern Syria, where there is a coalition training base for local forces fighting ISIS.”

Russia has now declared an end to cooperation to prevent air clashes over Syria and asserted an intent to track and target aerial intruders in its area of operations west of the Euphrates.

Such targets would be U.S. planes and surveillance drones.

If Moscow is not bluffing, we could be headed for U.S.-Russian collision in Syria.

Sunday’s shoot-down of a hostile aircraft was the first by U.S. planes in this conflict. It follows President Trump’s launch of scores of cruise missiles at a Syrian airfield in April. The U.S. said the airfield was the base of Syrian planes that used chemical weapons on civilians.

We are getting ever deeper into this six-year sectarian and civil war. And what we may be witnessing now are the opening shots of its next phase — the battle for control of the territory and population liberated by the fall of Raqqa and the death of the ISIS “caliphate.”

The army of President Bashar Assad seeks to recapture as much lost territory as possible and they have the backing of Russia, Iranian troops, Shiite militia from Iraq and Afghanistan, and Hezbollah.

Assad’s and his allied forces opposing ISIS are now colliding with the U.S.-backed Syrian Democratic Forces opposing ISIS, which consist of Arab rebels and the Syrian Kurds of the PYD.

But if America has decided to use its air power to shoot down Syrian planes attacking rebels we support, this could lead to a confrontation with Russia and a broader, more dangerous, and deadly war for the United States.

How would we win such a war, without massive intervention?

Is this where we are headed? Is this where we want to go?

For, again, Congress has never authorized such a war, and there seems to be no vital U.S. interest involved in who controls Raqqa and neighboring lands, as long as ISIS is expelled. During the campaign, Trump even spoke of U.S.-Russian cooperation to kill ISIS.

While in Saudi Arabia, however, he seemed to sign on to what is being hyped as an “Arab NATO,” where the U.S. accepts Riyadh as the principal ally and leader of the Gulf Arabs in the regional struggle for hegemony with Shiite Iran.

Following that Trump trip, the Saudis — backed by Egypt, the UAE and Bahrain — sealed their border with Qatar, which maintains ties to Iran. And though Qatar is also host to the largest U.S. air base in the region, al-Udeid, Trump gave the impression its isolation was his idea.

President Trump and his country seem to be at a decision point.

If, after the fall of ISIS in Raqqa, we are going to use U.S. power and leverage to solidify the position of Syrian rebels and Kurds, at the expense of Damascus, we could find ourselves in a collision with Syria, Russia, Hezbollah, Iran and even Turkey.

For Turkish President Erdogan looks on our Kurdish allies in Syria as Kurdish allies of the terrorist PKK inside his own country.

During the campaign, candidate Trump won support by pledging to work with Russia to defeat our common enemy. But if, after ISIS is gone from Syria, we decide it is in our interests to confront Assad, we are going to find ourselves in a regional confrontation.

In Iraq, the U.S. and Iran have a common foe, ISIS, and a common ally, the government in Baghdad. In Syria, we have a common foe, ISIS. But our allies are opposed by Assad, Russia, Iran and Hezbollah.

The question before us: After Raqqa and Mosul fall and the caliphate disappears, who inherits the ISIS estate?

The U.S. needs now to delineate the lines of advance for Syria’s Kurds, and to talk to the Russians, Syrians and Iranians.

We cannot allow our friends in the Middle East and Persian Gulf to play our hand for us, for it is all too often in their interests to have us come fight their wars, which are not necessarily our wars.

Patrick J. Buchanan is the author of a new book, Nixon’s White House Wars: The Battles That Made and Broke a President and Divided America Forever.