The problem the United States has always had in crafting a Syria strategy is that Washington never possessed sufficient leverage to ameliorate Bashar al-Assad's behavior. Providing arms to opposition groups provided some leverage (but was always fraught because of the lack of reliability of such groups), as did financial and personal sanctions. But prioritising the fight against Islamic State, exerting pressure on the Assad regime, and assisting the Iraqi government in re-establishing control across the border has proven a difficult set of tasks to accomplish simultaneously.
Meanwhile, Iran and Russia have (and always have had) much greater leverage in Syria. The Soviet Union was the major arms supplier and adviser of the Syrian Arab Armed Forces, while the number of Shia shrine sites in Syria (in Damascus and Raqqa in particular), and the need for a logistics hub for Hizbullah, saw Iranian money and pilgrims enter Syria in large numbers. Iran and Russia have also had much greater commitment to the regime than Washington has been able to muster for the potpourri of rebel groups. It therefore comes as little surprise that announcements over the last few weeks have reinforced the disparity between Iranian and Russian influence in Syria and that of the United States and other Western states.
Russia has had an arrangement with the Syrian government for its naval base at Tartus since 1971 and a bilateral security treaty since 1980 (the agreement with the Soviet Union transferred to Russia in 1992). The agreement the two countries signed in January this year that gave Russia a 49-year lease on Tartus, and allowed for the doubling of its berthing capacity for Russian vessels, (as well as a formal long-term arrangement for the Russian Khmeimim airbase in Latakia) should be seen as an extension of a long-standing close relationship.
The timing of the announcement (just before the US presidential inauguration and the talks with rebel groups in Astana) was pure theatre. It signaled to Washington, the rebel groups corralled at the table in Kazakhstan, and to countries such as Turkey that Moscow was committed to Syria for the next 50 years, just as it had been for the last four decades, and that its commitment was predicated on the continued existence of the government with whom it had signed the agreement.
Meanwhile, Iran continued to exact economic advantage from the Assad regime in exchange for the blood and treasure it has invested in his government's survival. Tehran has already given Damascus more than $4.5 billion in lines of credit since 2013 and has again extended a line of credit - reported to be around $500 million - for the import of basic foodstuffs. Nothing however, ever comes for free: Tehran is becoming increasingly involved in the Syrian economy, from phosphate mining contracts and leasing of agricultural land to, perhaps most importantly, a licence to operate mobile phone services in Syria.
Conducting a brutal cost-benefit analysis of US interests in Syria, it should be apparent to the Trump administration that Russian and Iranian sunk costs in maintaining the Assad regime are significant, while Washington's are minimal. It will take a massive investment of resources to change that dynamic and there's little indication of the requisite willpower. For all the talk of as-yet undefined 'safe zones', and cooperating with Russia against Islamic State, there's little indication that Washington has any inclination to mount a hostile takeover of Russian and Iranian assets in Syria.