Models for possible Syria intervention
Western states have been edging towards more direct support for President Bashar al-Assad's opponents, with the US now saying it will provide military aid to rebels. Here, Jonathan Eyal of the Royal United Services Institute looks at recent examples of Western military action that could provide models for intervention, if diplomacy fails.
US forces were involved in a number of international military operations over the past two decades, from the large-scale intensive warfare of the 1991 Gulf War down to the very small - if albeit tragic - US involvement in Somalia.
In all cases, humanitarian concerns were put forward as a key justification, but the real reasons why the US decided to act usually involved broader considerations, such as maintaining regional stability or upholding the cohesion of Nato and other Western-led military alliances.
The US also used secret arms supplies to rebels and no-fly zones as key policy instruments even if these ran contrary to international law restrictions, but these were only decisive when combined with a broader US military engagement.
Codenamed Operation Desert Storm, the 1991 Gulf War is still considered as a perfect case-study in international intervention, conducted by a US-led global military coalition, fully anchored in international law and boasting an explicit mandate from the UN Security Council, in pursuit of clear and limited objectives.
That was partly the result of Saddam Hussein's egregious behaviour: had the Iraqi leader refrained from occupying and annexing all of Kuwait's territory, it is likely that the international response would have been more feeble.
And the broader strategic environment also helped: with the Cold War just ended, the Soviet Union was desperate to forge a new partnership with the West, and the moral authority of the US as the supposed "winner" of the 20th Century's ideological contests was at its zenith.
Marshalling the required war resources proved relatively straightforward. The anti-Iraq coalition attracted almost one million soldiers from 12 contributing nations, with half of the troops from the US. A further 27 countries offered logistical and financial support.
The war achieved its immediate objectives. Iraqi forces were evicted, and Kuwait's sovereignty was restored.
More importantly, the US resisted the urging of some of its Arab allies who wanted to continue the fighting deep into Iraqi territory in pursuit of "regime change", precisely because the Americans feared that this would shatter the international consensus and spirit of security co-operation which flourished at that time.
However, hopes were dashed that the operation would provide a template for subsequent military interventions, thereby promoting respect for international law and security, because Desert Storm was an old-fashioned, classic war between sovereign states over ownership of a precise bit of territory.
As such, it offered few useful lessons for the management of subsequent crises, which usually entailed civil wars fuelled by the internal collapse of fragile states.
Meanwhile, the post-Cold War momentum for international co-operation also evaporated. Just one year after Desert Storm, Russia and the US were again at loggerheads over the legality of international interventions in the Balkans.
A series of Yugoslav wars began soon after Slovenia and Croatia declared their independence from the other republics of Yugoslavia on 25 June 1991, and lasted in one way or another for the remainder of the 1990s.
When efforts to prevent Yugoslavia's violent break-up failed, European leaders insisted that this was "their hour", and they should be allowed to handle the crises in their backyard alone.
The US administration of President Bill Clinton readily acquiesced, but it quickly became clear that the Europeans were both unwilling and unable to prevent bloodshed in the Balkans.
Croatia's territory was torn apart by a rebellion of ethnic Serbs, and a vicious war erupted in Bosnia-Hercegovina, another republic which seceded from Yugoslavia in March 1992.
European nations - with Britain and France in the lead - were increasingly drawn in, unwilling to take part in the war but unable to keep the peace.
The outcome was that they watched helplessly as the siege of Sarajevo throttled the Bosnian capital, and thousands were massacred in places such as Srebrenica in July 1995, right under the noses of UN peacekeepers.
It ultimately took the US to overturn this depressing spiral of violence, largely by supplying the anti-Serb resistance in both Croatia and Bosnia with weapons, in defiance of a UN-mandated embargo.
That helped turn the pressure on the Serbs, but only because it was accompanied by a US-led air campaign against Serb paramilitaries that paved the way for the imposition of the US-negotiated Dayton Agreement, ending the Bosnia war in November 1995.
US jets also provided the bulk of the 38,000 sorties that Nato conducted against Serbia between March and June 1999, in an effort to prevent massacres in Kosovo, then an ethnically-Albanian province of Serbia.
In both Bosnia and Kosovo, US air power failed to dislodge Serb forces or destroy the Serbian military.
Still, the Serbs were forced to accept US-dictated settlements to their wars, largely because they were alone: Russian support for Serbia remained confined to rhetoric, unlike Iranian support for today's Syria, which consists of both weapons and battle-hardened fighters.
The US also eschewed grand objectives, like the recreation of Yugoslavia or regime change in Serbia: the purposes of the military operations in the Balkans were, therefore, more realistic that those currently pursued in Syria.
The Kosovo operation remained legally controversial. Although the UN Security Council expressly linked its resolutions demanding the end of violence in Kosovo to "enforcement measures" under the UN Charter, Russia denied that this gave either the US or Nato the right to use force.
By the late 1990s, however, the Europeans were so exhausted by the seemingly unending crises in the Balkans that they accepted both US military leadership and Washington's legal justifications.
Still, throughout the Yugoslav wars the Europeans supplied most of the troops on the ground in the Balkans - the US contribution was overwhelmingly in terms of air power and precision munitions.
Faced with a humanitarian disaster precipitated by a complete failure of the Somali state, the UN Security Council authorised in December 1992 the creation of an international force with the aim of facilitating humanitarian supplies.
The US was not initially involved in this effort and had little at stake in the Horn of Africa. However, the Americans gradually started contributing to the Somalia operation, partly because it was judged to deserve international support, but largely because US President Bill Clinton wished to deflect criticism about America's refusal to contribute to UN-led operations in Yugoslavia at that same time.
The result was, however, a creeping US military involvement without a clear objective which culminated in disaster: The deaths of 18 US servicemen during the Battle of Mogadishu - more commonly referred to as Black Hawk Down - on 3 and 4 October 1993.
The tragedy had an immediate impact on American public opinion.
US troops were hastily withdrawn, though the civil war in Somalia continued.
The Somalia episode has gone down in the Pentagon's strategic annals as a classic example of how not to conduct an international operation.
It was not the US, but France and Britain that sought UN Security Council authorisation for a humanitarian operation that they said was needed to save the residents of the rebel city of Benghazi from being massacred by forces loyal to Libyan leader Muammar Gaddafi.
Nevertheless, the adoption of UN Security Council Resolution 1973 on 17 March 2011 that provided the justification for military action would not have been possible without the diplomatic lobbying of US senior diplomats, who succeeded in persuading both Russia and China to abstain rather than veto the resolution.
Nor would the subsequent air campaign have succeeded without the logistical, intelligence and ordnance support provided by the US.
European commentators derisively accused the US of "leading from behind" in Libya by refusing to put American soldiers in harm's way.
Yet it was the US that bore the biggest military burden for an operation that took much longer than anyone envisaged before Muammar Gaddafi was killed on 20 October 2011.
Russia, however, has protested ever since that the initial UN Security Council resolution only authorised the creation of a no-fly zone over Libya, and not a blanket, Nato-led bombardment that continued for months.
The Russians were also angered by the fact that, although all Western governments denied any intention to promote "regime change" in Libya, in practice the Western military offensive continued precisely until Gaddafi was overthrown.
The dispute over Libya led directly to the current diplomatic impasse over Syria.
Russia is determined to veto any UN resolution almost regardless of its content, largely because it fears that, as happened during the 1999 Kosovo crisis and the 2011 Libya showdown, the US and its Western allies would misinterpret these resolutions as a blank cheque to do as they please.
The Russians are also threatening to supply S-300 surface-to-air missiles to Syrian government forces should Western government begin supplies of weapons to Syrian rebels, precisely because Moscow is no longer willing to be out-manoeuvred by illicit Western weapon deliveries, as it was during the Balkan wars of the 1990s.