(CNN) A new phase of the Syrian conflict has begun, one in which outside powers are stepping up their involvement as the pressure gathers on ISIS' last redoubts.
Two events in recent days -- the shooting down by a US plane of a Syrian Su-22 and the use of ballistic missiles by Iran against ISIS targets -- are evidence of a scramble in eastern Syria that's been gathering pace since the beginning of the year.
It's the first time the United States has shot down a Syrian military aircraft, and at least the fifth occasion it has targeted regime and pro-regime forces since the Trump administration took office.
On Tuesday, a US fighter jet shot down a pro-Syrian regime drone in the country's southeast, US officials told CNN.
A lot of forces with competing aims are at close quarters in eastern Syria. The United States is aggressively backing a Syrian rebel alliance -- the Syrian Democratic Forces (SDF) -- as they fight their way into Raqqa, ISIS' administrative capital for the past three years. Hundreds of US military advisors are close to the front-lines, supported by intense coalition airstrikes.
The Syrian army and its allies (largely Iranian, Iraqi and Lebanese Shia militia), however, are also closing in on Raqqa. Last week the Syrian military reached areas controlled by the SDF. It was almost inevitable that at some point these opposing alliances would butt heads. So when the Syrian air force bombed SDF positions Sunday, the US came to the aid of its partners on the ground -- and the Syrians have one fewer Su-22.
The Pentagon said the action was "in accordance with rules of engagement and in collective self-defense of coalition partnered forces," but that was not the view in Moscow.
Russia announced Monday that for the second time this year it was suspending its military cooperation agreement with the United States in Syria -- an agreement designed to prevent unintended accidents in the skies over this crowded neighborhood. But it went a step further, warning that anything in the air west of the Euphrates River -- including coalition aircraft -- would be considered a target.
On Tuesday, Australia -- a US coalition partner -- announced that it was suspending all air operations over Syria.
The Russian Defense Ministry had already complained that the US and its supporters were allowing ISIS fighters to escape from Raqqa; Russian airpower and cruise missiles have been very active in the past month going after ISIS convoys and encampments in the desert south of Raqqa.
Russian Deputy Foreign Minister, Sergey Ryabkov, described Sunday's shoot-down as an act of aggression by the US and "an act of assistance to those terrorists whom the United States is ostensibly fighting against."
The cooperation with Russia against international terrorism that US President Donald Trump once advocated looks a distant prospect; the contest for control of eastern Syria is now in earnest.
Further east, the Syrian regime wants to regain the city of Deir Ezzor, most of which is still under ISIS control. Strategically located on the road to Iraq, and with a large airbase, Deir Ezzor and the towns around it seem likely to be ISIS' Alamo.
The Iranian missile attacks on ISIS positions around Deir Ezzor Sunday were in retaliation for the terror attacks in Tehran earlier this month, but they also serve a larger purpose. Iran's ultimate aim is to create a land corridor through Iraq and into Syria and Lebanon -- projecting its influence towards the Mediterranean.
Its allies in Iraq, the Shia militia known as Popular Mobilization Units, have already parked themselves on the Syrian border, clearing ISIS out of towns like Ba'aj in the process.
And so the battle is moving inexorably towards a band of territory where the main highways linking Iraq and Syria are located. According to an assessment by the US Defense Intelligence Agency in May, ISIS is "moving key leaders and functions out of Mosul and Raqqa, relocating them to safe havens along the Euphrates River in Syria and Iraq." Safe havens like the town of Mayadin that are already becoming distinctly unsafe for ISIS' beleaguered remnants.
The US has established two small bases along the Iraq-Syria border in an effort to stop the Syrian regime and Iranian-backed militia from controlling the area. But those bases are isolated -- and last week the Syrian regime trumpeted the arrival of its troops on the border for the first time in three years. Someone else also turned up -- the commander of the Iranian Quds Force, Major General Qassem Suleimani.
There is still fighting elsewhere in Syria, notably in the south around Daraa. But many rebel factions have sued for peace or been shunted into Idlib in north-western Syria.
For now at least the main contest lines up the regime of Bashar al Assad with its allies in Moscow and Tehran against the United States and the few remaining non-jihadist groups still on the Syrian battlefield.
More is at stake than the liquidation of ISIS.