After four devastating years of war in Syria, the tide has turned against the vicious Assad dictatorship. Since March, the regime has suffered a string of defeats across the country. The seizure of Palmyra by the ultra-reactionary Islamic State (ISIS) grabbed headlines in May, but more significant are the gains by genuine rebel forces in other areas.

After a succession of successful offensives, Idlib province in the north-west is now almost entirely in the hands of an alliance of rebel groups. In the south, Free Syrian Army fighters in Daraa province have made significant gains. Regime forces are on the verge of being expelled from Aleppo in the north, although rebel advances there are now threatened by an ISIS offensive against the city.

 “This is not just the ebb and flow of battle”, one Arab diplomat told the Guardian. “This has been clear and repeated evidence that the regime army cannot defend itself, or the country, even with the ever-heavier backing of its sponsor, Iran.”

The regime is abandoning any pretence of waging a war across the whole country. Instead it is retreating to a strategy of defending its core areas in and around Damascus, and the Alawite heartland near the Mediterranean coast.

The other part of the regime strategy, which fits with the policy it has adopted since the beginning of the war, is to back ISIS when it comes into conflict with rebel forces.

The latest example of this is the battle for Aleppo. ISIS has begun a sustained offensive against rebel positions in the city, backed by regime airstrikes that rebel leaders say show clear signs of coordination between ISIS and the regime.

EA worldview reports: “The Assad air force [had been] bombing Islamic State-held areas in Aleppo Province. However, as soon as the Islamic State challenged rebels along a 25km front north and northeast of Aleppo city – pulling back from their threat to regime positions to the east – the Syrian aerial assault focused on rebel territory”.

The US, which claims to be fighting a war against ISIS, has pointedly refused to extend its bombing campaign to areas where Syrian rebels are fighting the Islamic State, in spite of pleas from rebel leaders.

On 29 May, rebel commander Zaher al-Saket told the US website Daily Beast: “We were hoping that we could work hand-in-hand with coalition forces to defeat ISIS and that the coalition would launch strikes against ISIS-held positions in north-east Aleppo. We called on them to do so”.

In the past, the US has refused to bomb in support of rebel groups fighting ISIS on the basis that this would help Jabhat al-Nusra. This time, the excuse was even more cynical. According to al-Saket, “We were rebuffed for the astounding reason that aiding the rebels in Aleppo would hurt Assad, which would anger the Iranians, who might then turn up the heat on U.S. troops in Iraq”.

The US claims to oppose both Assad and ISIS. In reality, it is assisting Assad in his last gasp attempt to hold off the rebel advance by strengthening ISIS across northern Syria. You really couldn’t make it up.

While the US continues to refuse assistance to the Syrian rebels, a different approach is being taken by key regional powers.

An important factor facilitating rebel advances has been the sudden availability of weapons via Saudi Arabia and Turkey. In March, a meeting in Riyadh convened by the new Saudi king, Salman, and attended by Turkish president Erdogan and officials from Qatar and the Gulf Cooperation Council, resolved that they would finally give rebel groups in the north (excluding ISIS) the weapons they have been pleading for since 2011.

Within weeks, arms began to flow to the opposition. “I could now get nearly all the weapons I wanted”, one rebel commander told the Observer. “For the first time they were not holding anything from us – except anti-aircraft missiles. The Turks and their friends wanted this over with.”

These developments are of course grist to the mill for those “leftists” who have dedicated themselves to the defence of the fascist Assad regime and its barrel bombs and torture cells. From the beginning of the armed struggle in 2011, such people have accused the opposition of being pawns of outside powers.

The reality is that such support for the opposition has been, until now, paltry at best. The real “foreign intervention” into the Syrian war has been billions of dollars in military support given to the regime by Iran and Russia. Over the past year, Iranian militias under the command of Iranian military officers, along with Hezbollah units, have increasingly taken over as the leading force defending Assad’s rule.

The new flow of arms to the rebels, and the dramatic shift that has resulted on the battlefield, in fact gives the lie to the claim that the opposition have been armed by outside powers all along. If they had been, Assad would be long gone. What we are now seeing is what happens when both sides of the conflict have substantial outside backing: the rebels make stunning gains, and the regime is isolated and faces growing dissent among its own supporters.

The decision by Turkey and Saudi Arabia to arm rebel groups is of course utterly cynical. King Salman is no more interested in democracy and freedom in the Middle East than his predecessor, Abdullah. The new Saudi regime, alarmed at the rapprochement between Tehran and Washington, is pursuing a new strategy of Saudi assertiveness to try to push back against what it perceives as growing Iranian power.

Arming Syrian rebels and bombing Yemen back to the Stone Age are two prongs in a single Saudi strategy to impose itself as the leader of the Arab world and assert its ability to act independently of the US.

Syrian rebels have every right to take advantage of this situation to obtain the weapons they desperately need to bring down the regime.

Overthrowing Assad will not solve all of Syria’s problems. Years of war have torn Syria apart, displacing millions and killing hundreds of thousands. That reality, combined with the retreat of the revolutionary wave across the Arab world more broadly, means that much of the original social content of the 2011 uprising has been lost.

The rise to prominence among the rebels of reactionary Islamist groups like Jabhat al-Nusra is no small problem. But the first problem remains the regime. Without its overthrow, Syria has no future but war or totalitarianism.