The evolution of Syria's conflict is sharpening discussion of western policy towards the rebels, in particular whether the United States should supply them with arms. At the same time, many analysts warn that the war could spread to other countries in the region, citing especially Lebanon, Turkey or Jordan.

Iraq is an often neglected dimension of both issues. For the recent upsurge of violence in that country reflects in part the spillover effects of the Syrian conflict, and in turn compounds Washington's reluctance to get further involved in Syria. The Iraq factor may yet exert a great influence on the unfolding regional dynamics (see "Iraq, Iran, Syria: triangle of war", 9 August 2012).

The trend of conflict

This week the Pentagon reported to Congress on options for United States intervention. The context is an acknowledgment by the US and Britain that the Assad regime is relatively robust and unlikely to collapse any time soon (see Mark Landler & Thom Shanker, “Pentagon outlines range of Syria options”, New York Times, 24 July 2013). General Martin E Dempsey presented a range of possible options, including aggressive ones (no-fly zones and direct attacks on regime forces). Any such action would be an act of war, with long-range strikes on military targets requiring “hundreds of aircraft, ships, submarines and other enablers” with costs “in the billions”.

Dempsey said that this approach “would likely further the narrow military objective of helping the opposition and placing more pressure on the regime." But he also warned: "Once we take action we should be prepared for what comes next. Deeper involvement is hard to avoid.”

The general's caution is unwelcome to Republican hawks eager for quick action. It does, though, reflect the administration's view, which is rooted partly in an analysis of the evolution of Syria's rebellion. This has been marked by the strengthening of Assad's forces and the growing power of more radical Salafist elements among the insurgents. The largest of the radical groups may be Ahrar al-Sham, with 10,000-20,000 fighters; it seeks Islamist governance within Syria, and has little connection with the wider al-Qaida movement. By contrast Jabhat al-Nusra, more clearly linked to al-Qaida (and with a higher profile), has perhaps 7,000 fighters (see "Competition among Islamists”, Economist, 20 July 2013).

Jabhat al-Nusra has had close links with Salafist paramilitaries in Iraq, and though these have become strained the group remains active, especially in northeast Syria. Beyond their differences of focus, more significant is these two groups' capacity. They probably can mobilise over 20,000 paramilitaries, and are better organised and more efficient (if also brutal) when they take territory - in contrast to more secular armed groups, which have acquired a reputation for looting and disorder.

The risk of escalation

The overarching issue is the combination of numbers and effectiveness. In early 2012, external assessments were that the Salafists had under 2,000 fighters, a figure now perhaps expanded tenfold. If western military intervention to help terminate the Assad regime were to happen, it could eventuate in the Salafists dominating a post-Assad Syria: not an encouraging prospect for advocates of intervention (see "Syria, al-Qaida, and the future", 2 August 2012).

An additional factor is the rise of al-Qaida affiliates in Iraq. A wave of anti-government and anti-Shi'a bombings and shootings have reached an intense pitch, and is marked by some spectacular actions. Late on 21 July 2013, for example, the Abu Ghraib and Taji prisons in central Iraq were attacked in a massive paramilitary operation which released around 300 prisoners, many of them experienced fighters; at least fifty people were killed in the assault.

The al-Qaida-orientated groups in Iraq have benefited from Sunni opposition to Nouri al-Maliki's government, which they see as consistently favouring the Shi'a majority and supporting the Assad regime's repression of Sunnis (not least by funnelling Iranian supplies through to Syria).

This highlights the shift towards what is becoming a transnational intra-confessional conflict. The Shi'a-related Alawis in Syria are supported by the Shi'a majority regime in Iraq and by Iran; the largely Sunni insurgents in Syria are supported by Qatari and Saudi interests, while many of them also make common cause with the evolving insurgency in Iraq. The connections between the fighting in Syria and Iraq grow stronger by the week.

The slim hope

At an elite level, there is a transnational stalemate. If Assad grows stronger, Salafist elements of the rebels will get more support from the Gulf regimes, especially Saudi Arabia, and the war will go on; if the rebels grow stronger, Assad will get more support from Iraq and Iran, and the war will go on - while Iraq itself moves closer to civil war.

The human costs of the Syrian war, meanwhile, are enormous: around 100,000 people killed, many tens of thousands wounded, 1.9 million people displaced across international borders to Lebanon, Turkey, Iraq, Jordan and Egypt, many hundreds of thousands displaced within the country.

There is one possible route to progress in alleviating this political and humanitarian crisis , as several columns in this series have argued: in a determined effort by Washington and Moscow to work together to limit the actions of the regional proxies (see "Syria, the last chance", 6 December 2012). The good personal relationship between Russian foreign minister Sergei Lavrov and US secretary of state John Kerry may be a positive sign here, as is the more open attitude of Iran's new president Hassan Rowhani; but overall, the chances for real movement - beginning with an international peace conference - are limited.

Could this change? An escalation of violence in Iraq into something approaching a civil war might just be enough to persuade Riyadh, Doha and Tehran to stand back, and Kerry and Lavrov to be bold. In the absence of diplomatic movement, Syria's war will go on and on, with the increasing risk that it is transformed into a much wider war that fuses several intra-state conflicts and spreads across the region. This unholy outcome alone should prompt those with the power to avoid it to take risks for peace.