Gayle Tzemach Lemmon is a senior fellow at the Council on Foreign Relations. She is the author of "Ashley's War: The Untold Story of a Team of Women Soldiers on the Special Ops Battlefield." The opinions expressed in this commentary are hers.

(CNN) News that a US Navy fighter jet on Sunday shot down a Syrian warplane is a dramatic escalation in a war the United States had not intended to fight.

It's also a development that's been years in the making.

At the same time, Iran fired missiles into Syria over the weekend. On Monday, Russia said it has ceased its military cooperation with the United States in Syria over the downing of the plane, referring to the act as "military aggression."

These events illustrate how an uneasy battlefield coexistence in Syria has spiraled into a situation that is impossible to maintain. Now come the tasks of calming an escalating confrontation with Syrian President Bashar al-Assad along with his Iranian and Russian backers and working to return the focus to the war in Syria in which the US does want to participate -- the battle against ISIS.

"The Coalition's mission is to defeat ISIS in Iraq and Syria," a spokesman for Operation Inherent Resolve stated in a release. "The Coalition does not seek to fight the Syrian regime, Russian, or pro-regime forces partnered with them, but will not hesitate to defend Coalition or partner forces from any threat. The Coalition presence in Syria addresses the imminent threat ISIS in Syria poses globally. The demonstrated hostile intent and actions of pro-regime forces toward Coalition and partner forces in Syria conducting legitimate counter-ISIS operations will not be tolerated."

This is a moment that was nearly inevitable, as the US has charged the military with pursuing a series of on-the-ground tactics to defeat ISIS without an overarching US policy for Syria. As US Secretary of Defense James Mattis noted, it is currently a " strategy-free " time.

Indeed, since 2011, those who wanted to avoid greater intervention in the battle against Assad feared that it would drag the US, step-by-step, into another war in the Middle East. And until now the overarching US guiding principle has been to avoid such direct conflict with Assad forces on the ground -- at nearly all costs.

Yet here we are. At a moment in which the US has been using "de-confliction" lines to avoid a conflict with Russia, the fight against ISIS is taking the US ever-nearer to a head-on collision with the Syrian regime, as forces allied with Assad target forces backed by the US.

For years, Washington debated the idea of safe zones inside Syria to avoid further civilian carnage in a war now estimated to have killed nearly half a million people.

And for years the idea was discarded, in no small part because the US did not want to find itself in direct conflict with the Syrian regime.

"The time has come for President Assad to step aside," might have become official United States policy as of 2011 . But Assad can stay for now had become de facto guidance as the US sought to avoid direct intervention in the Syrian conflict and an endless series of Geneva conferences aimed at a diplomatic solution to the war took place.

The ghost of the Iraq war hung over every decision on Syrian intervention, and the risk of taking incremental steps that might lead the US into another ground war in the region guided the Obama administration's decision to stay out of the escalating conflict. So the US supported a program to arm moderate rebels , but would never state just how far it would go to protect them if they were targeted by regime forces.

The US hunted for the Goldilocks strategy on the war in Syria and ended up doing enough to help rebels fighting Assad around to the very periphery, but far from enough to be decisive in the war.

Even after Russia went all-in on the side of the Syrian President to devastating effect -- most dramatically in the city of Aleppo -- the US sought to stay out of a country-to-country confrontation with either the Syrian regime or its generous backers in Russia and Iran.

Indeed, in the last few years ISIS has surpassed Assad and become the threat driving US military intervention in the country. American focus has since been squarely on the fight against ISIS, not the regime.

And yet now, as the Trump administration enters its sixth month, the US is being drawn into the very conflict its inaction had been intended to avoid. And as forces the US supports face danger from forces supporting the Syrian regime, the questions will get louder: What is US policy in Syria? And will the fight against ISIS lead the US into a war against Assad?

The questions come at a pivotal time in the campaign to retake the ISIS stronghold of Raqqa -- and as questions about what will follow the fall of ISIS remain to be answered.

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Now will come more questions, more uncertainty as the country waits to see how US policy on Syria evolves as the facts on the ground change.