The Chinese primarily provided political cover, keeping the Russians from having to operate alone diplomatically. They devoted no resources to the Syrian conflict but did continue to oppose sanctions against Iran and provided trade opportunities for Iran. The Russians made a much larger commitment, providing material and political support to the al Assad regime.
It seems the Russians began calculating the end for the regime some time ago. Russia continued to deliver ammunition and other supplies to Syria but pulled back on a delivery of helicopters. Several attempts to deliver the helicopters "failed" when British insurers of the ship pulled coverage. That was the reason the Russians gave for not delivering the helicopters, but obviously the Russians could have insured the ship themselves. They were backing off from supporting al Assad, their intelligence indicating trouble in Damascus. In the last few days the Russians have moved to the point where they had their ambassador to France suggest that the time had come for al Assad to leave -- then, of course, he denied having made the statement.
A Strategic Blow to Iran
As the Russians withdraw support, Iran is now left extremely exposed. There had been a sense of inevitability in Iran's rise in the region, particularly in the Arabian Peninsula. The decline of al Assad's regime is a strategic blow to the Iranians in two ways. First, the wide-reaching sphere of influence they were creating clearly won't happen now. Second, Iran will rapidly move from being an ascendant power to a power on the defensive.
The place where this will become most apparent is in Iraq. For Iran, Iraq represents a fundamental national security interest. Having fought a bloody war with Iraq in the 1980s, the Iranians have an overriding interest in assuring that Iraq remains at least neutral and preferably pro-Iranian. While Iran was ascendant, Iraqi politicians felt that they had to be accommodating. However, in the same way that Syrian generals had to recalculate their positions, Iraqi politicians have to do the same. With sanctions -- whatever their effectiveness -- being imposed on Iran, and with Iran's position in Syria unraveling, the psychology in Iraq might change.
This is particularly the case because of intensifying Turkish interest in Iraq. In recent days the Turks have announced plans for pipelines in Iraq to oil fields in the south and in the north. Turkish economic activity is intensifying. Turkey is the only regional power that can challenge Iran militarily. It uses that power against the Kurds in Iraq. But more to the point, if a country builds a pipeline, it must ensure access to it, either politically or militarily. Turkey does not want to militarily involve itself in Iraq, but it does want political influence to guarantee its interests. Thus, just as the Iranians are in retreat, the Turks have an interest in, if not supplanting them, certainly supplementing them.
The pressure on Iran is now intense, and it will be interesting to see the political consequences. There was consensus on the Syrian strategy, but with failure of the strategy, that consensus dissolves. This will have an impact inside of Iran, possibly even more than the sanctions. Governments have trouble managing reversals.
From the American point of view, al Assad's decline opens two opportunities. First, its policy of no direct military intervention but unremitting political and, to a lesser extent, economic pressure appears to be working in this instance. More precisely, even if it had no effect, it will appear that it did, which will enhance the ability of the United States to influence events in other countries without actually having to intervene.