From all-out war to vulnerable pairs of military observers in blue berets, the United Nations Security Council can authorize a full spectrum of intervention.
Observers, peacekeepers, peacemakers – the terms vary as do the mandates. And there is plenty of overlap. Outcomes vary, too.
Some UN missions, such as the traditional buffer-zone peacekeepers still patrolling a "Green Line" in Cyprus after a half-century, seem to become part of an imperfect solution.
Others, like last summer's controversial bombing campaign over Libya, end relatively quickly but only after morphing, according to some key players at the Security Council, far beyond their original mandate.
In Syria, the current observer mission is mandated only to monitor a ceasefire that may never really take effect.
Even if all 300 UN military observers are in place soon, the mission's fate is already in doubt.
Most UN missions, like the one in Syria, require the "host" nation – in this case, Syria – to approve it. And thus Syria can toss the monitors out if it chooses, just as the UN can pull them if the mission is rendered ineffective.
Military observers, like those being sent to Syria, are usually mid-ranking officers who volunteer, on secondment from their "home" nations to the UN. They will likely operate in pairs or foursomes, moving in white-painted civilian SUVs. Although considered "unarmed," the observers will carry their handguns as part of their uniform – but they pose no military threat. Their only purpose is to observe. Given full freedom of movement, observer missions can be very effective. But without the active co-operation of both sides, and a clear commitment by the combatants to make a ceasefire work, the monitors have neither means nor authority to compel compliance.
So far, in chaotic and violent Syria, the handful of observers have, at times, been obstructed and used as cover for outbreaks of violence.
Already, some want a more forceful intervention. France's Foreign Minister, Alain Juppé, has warned that if mediation – as monitored by the thinnest of blue lines, a mobile UN observer force – fails, then the Security Council must consider more aggressive military options.
"We cannot allow ourselves to be defied by the current regime," Mr. Juppé said.
Syria, unlike Libya or Kosovo a decade ago, offers no relatively simple, militarily achievable outcome by outside powers. There are real risks that even a UN-mandated military intervention would escalate into a regional war and spill beyond Syria's borders.
At the UN Security Council, intervention in a member state takes two forms; they are referred to as Chapter Six and Chapter Seven, because that's where they appear in the UN Charter.
Broadly, Chapter Six intervention, such as monitoring a truce line, requires the agreement and participation of the parties involved. Chapter Six operations can't use force, except in self-defence. Peacekeeping, monitoring and observer forces in all their variations are Chapter Six operations.
Far more rare are Chapter Seven missions when the Security Council approves the use of military force – usually subcontracted to a group of states – to impose an outcome on an unwilling state.
The Korean War was Chapter Seven. So was the U.S.-led war to oust Iraq after its 1990 invasion of Kuwait. So was last year's Libyan campaign, although many – including importantly China and Russia – contend that the original limited mandate to impose a "no-fly" zone and protect the civilian population in Libya was unlawfully stretched so that NATO warplanes, including Canada's, became the de facto air force for rebels seeking to oust Colonel Moammar Gadhafi.
The blowback from Libya has so far meant that China and Russia, both veto-wielding permanent members of the Security Council, have made it clear they are flatly opposed to any type of "thin-edge-of-the-wedge" Chapter Seven mandate that could similarly be morphed into major military intervention in Syria.
Chapter Six operations vary widely. Some, like the current effort in Syria, are modest in size, firepower and mandate.
Others, like the long-running UN Protection Force in the Balkans in the 1990s, involved tens of thousands of "blue helmets" from more than a dozen nations, including Canada, with some units equipped with light tanks. But because it was a Chapter Six mandate, even those heavily armed units were severely constrained in the action they could take.
In effect, UNPROFOR peacekeeping troops largely watched the war rage around them and, in the case of the infamous Srebrenica enclave, surrendered to Serb forces who then massacred more than 8,000 Muslims supposedly sheltered in a UN "safe area."
As the international community grapples with what – if anything – to do next in Syria, a few stark realities remain.
Short of a full-blown enforceable military mandate – in effect a UN Security Council licence to wage war – there are no options unless Syria approves.
Some believe Syria's President Bashar al-Assad is just stalling. Over the past year, more than 9,000 people have died. Tanks, helicopter gunships and artillery fired on civilian neighbourhoods in Homs, Hamas and half a dozen other Syrian cities. Tens of thousands have fled to Turkey, far more have been displaced internally. While the al-Assad regime regards the violence as justified to suppress what it calls a terrorist effort to topple a legitimate government, much of the rest of the world sees it as a brutal repression of a pro-democracy movement.
President al-Assad isn't likely to approve any foreign force on Syrian soil that can tip the military balance against him. China and Russia seem implacably opposed to a Chapter Seven mandate that would authorize Western, and perhaps Arab, nations to topple the al-Assad regime. Humanitarian corridors and enclaves for the fleeing, while viscerally attractive, are notoriously difficult to implement without a believable threat of military enforcement.