Analysis

Western states, particularly the United States, have the military capability to defeat ISG swiftly in any ground intervention. However, they lack the political will and financial resources to deploy a substantial land force in the Middle East. In addition, they would want to have international legal backing from the UN Security Council -- which would require Russian support.

United States

The United States would be the critical component of any Western ground intervention. Its European and Gulf allies are unlikely to carry out any operation of this sort by themselves.

However, Washington will probably not reverse its current position against ground intervention. A major factor is popular opposition. In the wake of the Paris attacks, a Reuters poll of US nationals showed 76% opposed a US ground campaign.

Moreover, President Barack Obama -- who inherited wars in Iraq and Afghanistan -- is wary of handing his successor an unresolved military commitment with no clear pathway to a political solution (see US/RUSSIA: Obama will likely wait out Syria gambit - October 14, 2015).

There is support for more robust military action from more hawkish Democrats, such as presidential candidate Hillary Clinton or Secretary of State John Kerry, but they favour measures such as a Syria no-fly zone, special forces trainers to Syrian rebels, and increased air sorties -- not a conventional ground intervention.

Republicans have stridently criticised the White House for a "lack of leadership" in US foreign policy, but have not supported a radical departure from the current military strategy, such as a large-scale military intervention.

EU powers

Despite last month's Paris attacks, the French government does not want to send ground forces. It is convinced that this would be inefficient militarily, except in the unlikely event of a very large coalition and huge number of troops.

There is also some resistance among elites and opinion makers against the "logic of war", given the lack of strong military means that would allow for immediate military results. Paris is concerned that all the public will see is dangerous escalation.

No other EU states are able or willing to send ground troops. The UK parliament explicitly ruled out UK ground forces in its motion approving airstrikes on Syria. Germany's Bundestag votes tomorrow on the proposed German deployment of reconnaissance flights, a frigate and 1,200 troops to operate them, but a wider ground intervention is not on the agenda.

Turkey

Turkey might support a US-led ground operation in Syria, with troops and logistical support. The army is much more cautious in this respect than President Recep Tayyip Erdogan.

Turkey has been calling for a 'safe haven' on Syrian soil along the border, where Syrians fleeing the war could stay rather than adding to refugee numbers inside Turkey. It could use intervention to form a barrier against the Kurdish guerrillas of the Democratic Union Party (PYD) advancing westwards across the Euphrates at Jarabulus (see TURKEY/RUSSIA: Ankara will stick to its Syria policy - October 7, 2015 and see SYRIA: Turkish zone will push back 'Islamic State' - August 11, 2015).

However, the main obstacle to effective Turkish participation in any ground force is its sharply varying agenda in Syria compared with that of its NATO allies.

Turkey's priorities in Syria have always been the removal of the Assad regime and blocking the rise of the PYD, which is allied with the Kurdistan Workers' Party (PKK). It has only recently come to see ISG as an enemy, and had previously been operating a more permissive attitude to the group, viewing it as a useful counter-weight to PYD expansion.

Moreover, a military operation nominally against ISG could easily strengthen Ankara's Syrian enemies, the regime and the Kurds.

Saudi Arabia and the UAE

Saudi Arabia is heavily involved in Syria as one of the main external backers of Syrian rebel groups fighting the Assad regime. As the region's leading Sunni Arab power and a key US ally, Riyadh may be expected to play a key role in any Western- or international ground force fighting ISG.

However, this would be politically sensitive, as it would lay the Saudi leadership open to accusations from radical Sunnis at home and abroad of not only failing to defend Sunni Arabs in Iraq and Syria, but also joining in an international aggression against them.

In addition, Saudi Arabia's military is already heavily invested in the conflict in Yemen, and has poorly trained soldiers with little operational experience. The actual number of Saudi troops deployed inside Yemen is very low.

By contrast, the United Arab Emirates' (UAE) armed forces have performed strongly in Yemen, deploying over 3,000 ground troops. After a decade of heavy investment and military intervention alongside US forces in Afghanistan, the UAE has one of the best militaries in the region and is keen to project its military power (see GULF STATES: Yemen will demonstrate GCC military power - September 7, 2015) and (see UNITED ARAB EMIRATES: Foreign policy risks are rising - September 10, 2015).

UAE Minister of State for Foreign Affairs Anwar Gargash said this week that the UAE would participate in any international ground intervention against ISG.

Russia

Russian President Vladimir Putin's proposal for a broader international coalition against ISG has gained momentum in the wake of its intervention in Syria and the Paris attacks. This would bring Western states and their Gulf allies together with Russia, Iran and the Assad regime.

However, Russia's escalation has demonstrated that its priority in Syria is protecting the Assad regime rather than fighting ISG (see SYRIA: Russian intervention will empower jihadists - November 20, 2015). It has deployed substantial airpower to this end, but will try to hold off a major ground intervention for now.

Moscow is likely to judge that the Syrian army and Iranian-backed forces can do the ground fighting. Deploying Russian troops in a ground invasion is too risky and would also leave Russia further diplomatically isolated.

The continuing partnership with Tehran is therefore vital for the Kremlin, but Moscow may have to intervene with ground forces if the regime weakens further and Iran's support is not enough to prop it up(see IRAN: Syria partnership will deepen Russia ties - November 24, 2015).

Russia would probably be involved in any ground intervention if a broad international coalition to combat ISG is formed. The Kremlin would judge a ground intervention within a broad international collation backed by the UN as less risky militarily and politically it would aid Russia's rehabilitation following events in Ukraine.

Iran

Like Russia, Iran's priority is also ensuring the survival of the Assad regime in order to project its regional influence. Iran has provided military support to its longstanding ally in Damascus since the start of the war, deploying Islamic Revolutionary Guard Corps members and pro-Iranian Afghan and Iraqi militias.

The United States estimates that 2,000 Iranians are fighting in Syria, and appear to be taking an increasingly direct role in combat operations in the current Aleppo offensive.

Fighting ISG is also a priority for Iran, given the threat the group poses to Iran's allies in Syria and Iraq. Tehran would likely signal readiness to commit troops to any international ground force deployed to Syria, a gesture which would also benefit Iran's drive to normalise its diplomatic relations with the international community.

However, in practice there would be limited scope for deploying Iranian ground forces, due to the fact that they would be viewed as a hostile invasion force in the Sunni Arab areas in which ISG is primarily concentrated.