On the heels of a successful Geneva agreement between the P5+1 and Iran, the announcement on Monday that the peace conference for Syria's civil war – the so-called Geneva II conference – will be held on 22 January is welcome news. But even if all sides actually show up to the meeting, there is considerable doubt as to whether a political settlement between the warring Syrian parties is possible at this stage. To increase the chances of success, the US and Russia should pursue a ceasefire among the regional supporters of the war as a precursor to Geneva II. Let's call it "Geneva 1.5".

The conflict in Syria is no longer a domestic struggle. It has become a regional proxy war, principally between Saudi Arabia and Iran, but with important roles played by Qatar, Turkey and Iraq. These external actors are fanning the flames of conflict and actively dissuading their Syrian allies from committing to Geneva II.

The record on resolving such wars is clear. Until the external actors reach some sort of accommodation, they will continue to fund and arm their proxies and the war will continue indefinitely. We saw this unhappy dynamic frequently in civil wars that often lasted decades during the Cold War. In Iraq and Afghanistan, all of the might of the US military could not end civil wars fueled by outside powers. By contrast, in Lebanon in 1989, the regional supporters of the various sides of the civil war first had to agree to de-escalate before 15 years of bloody conflict could come to an end.

Thus far, no effort has explicitly addressed the role of regional actors in Syria and the conflicts between them. The Syrian factions would not be present as in Geneva II. Geneva I, which took place in July 2012, also did not include the Syrians but it focused on the principles of civil war resolution and excluded some of the key regional actors – particularly Iran and Saudi Arabia. The purpose of a Geneva 1.5 conference would be to facilitate eventual political resolution within Syria (executed in Geneva II) by first cutting off the activity of regional actors that fuels the conflict. The goal would be a ceasefire agreement.

The US and Russia could begin by bringing the key regional actors together to work on the question of humanitarian assistance in Syria and use that effort to move into discussions about the conflict and Syria's future. The very act of getting the Saudis and the Iranians around the same table to discuss Syria would be a major breakthrough, but once they're there, Washington and Moscow should push for genuine de-escalation. The key will be convincing all parties that they have little hope of realizing their maximalist goals and then finding a formula that can accommodate all sides' interests in a future Syrian settlement.

Despite the myriad difficulties associated with this approach, there are reasons to think that such a deal might be possible. In Syria, Iran is wasting precious resources on a struggle that it cannot win and in the process validating the Saudi narrative of the Sunni-Shi'a split, destroying its standing in the Arab world. Iran may accept a settlement that protects its core interests of ensuring its connection with Hezbollah and Lebanon and preventing Damascus from being controlled by a puppet government of another power. The success of the nuclear talks might also have made Tehran more likely to engage.

Meanwhile, the Saudis are facing the growing threat that Syria is becoming an incubator of a brand of al-Qaida-linked extremism that might eventually threaten their own rule. Their path to victory in Syria looks increasingly unclear as the Assad regime continues to demonstrate its resilience. Simply put, the Saudis do not have the capacity to win a long proxy war with Iran. Therefore, they might see the benefits of a power-sharing arrangement in Damascus that would give them some influence with a Syrian transitional government. Despite these incentives, Turkey and Qatar, which maintain somewhat better relations with Iran, would have to be enlisted to pressure Saudi Arabia to attend and negotiate.

The United States and Russia are not neutral parties in the Syrian war, but they are nonetheless best positioned to lead a Geneva 1.5. They share an interest in ensuring that Islamist extremists do not gain control of Syria. The US has closer relations with Saudi Arabia, Qatar, and Turkey and Russia has better ties with Iran. If Washington can demonstrate that it is serious about reining in its regional allies, Moscow might make a similar attempt to bring Iran to the table. Russia traditionally relishes the role of "guarantor" of the settlements to others' wars.

Convening a Geneva 1.5 would not be easy for Russia or the United States – there remains considerable distrust and differences between them over Syria. But as the chemical weapons deal demonstrates, US-Russia cooperation on Syria can pay significant dividends. Since agreement at Geneva II is probably beyond reach at the moment, pursuit of a regional ceasefire is the best option moving forward.