How will the West address Putin's moves in Crimea? With direct force already ruled out by Washington and Brussels, here are some alternatives, with pros and cons listed for each one. Doubtless there are others; feel free to add your own in the comments section below.

Broad-based economic and political sanctions against Russia, with the aim of isolating it from the international community

Pros: Sends a clear signal that the West will not tolerate military interventions unsanctioned by international legal bodies, no matter who does it.

Cons: Would be difficult to gather the will for it among EU members that have strong trading relations with Russia, and even harder to enforce. Energy would be off the table; sourcing gas elsewhere may hit the EU harder than Russia.

Likelihood: Low. While the US remains keen, Germany and the UK have already signaled reluctance.

Smaller 'targeted' sanctions against Russian interests

Pros: Sanctions can be much smarter today than the blunt instruments of the 1990s. Despite Kremlin blustering about dumping the US dollar, Russian banks are vulnerable. Visa restrictions on key business figures would also put pressure on Putin from within his own power base.

Cons: It worked against Iran, but Russia is bigger. Is unlikely to be anything more than a short-term strategy. May kick off a round of tit-for-tat reprisals.

Likelihood: High. It won't get Russia out of Crimea if it really wants to stay. But if Russia's oligarchs find themselves unable to travel to European capitals, expect them to make their displeasure with the Kremlin known. Putin doesn't control them as much as he used to.

A Western-led naval blocking force to quarantine Crimea

Pros: Sends a powerful message that if Putin can play with fire, the West can too. Crimea is economically weak, and a quarantine would make the situation there much more acute.

Cons: Would result in the partition of Ukraine by proxy, with the added spice of potential great-power war in the event of accidents. If any forces are deployed, NATO would have to do it, and many members would baulk at that.

Likelihood: Very low. Nobody wants a new militarised Cold War with the geopolitical boundaries shifted eastwards. There's already a 'Cold Peace' in any case.

Suspend Russian membership of various prestigious clubs

Pros: Moscow relies on prestige for much of its soft power. Denying Putin and senior bureaucrats the ability to be seen making deals in the G8 and Council of Europe gives the West some leverage. It also plays well to Western audiences.

Cons: Almost completely symbolic, and hasn't worked in the past. When the Council of Europe suspended Russia over its conduct in Chechnya, Russian diplomats were back within six months.

Likelihood: Low-medium, depending on the organisation. The Council of Europe is a real possibility, G8 less so.

And some wildcards…

Offer NATO membership to Ukraine

Pros: Resolves the post-Cold War 'Russia question' by simply containing Putin. Would be reassuring for a new Ukrainian government, and would also placate edgy Central and East European states.

Cons: Would drive Russia firmly into nationalistic anti-Westernism and closer to China. Would ignite an immediate gas war. Takes scarce US resources away form the Asia tilt. Oh, and Cold War redux.

Likelihood: Very low. The West is aiming for conflict de-escalation, not the reverse.

Offer NATO membership to Ukraine, the CIS states, Russia – and even China

Pros: The West has been reactive to Putin, and such a left-field move might wrest back the initiative. Says to Russia, China and anyone else that if they really fear the West, they can be part of it. NATO could hardly be more moribund in European security architecture at the moment anyway.

Cons: Every ex-communist member state would immediately block it. For them, NATO is entirely about keeping the Russians out. China and Russia could simply say ‘thanks, but no thanks’. Would also require a name change: any takers for ‘GLOBO’?

Likelihood: Very low indeed. But a major reorientation of regional and global security architecture will be needed at some point. While this is likely a non-starter, why not something similar – and on the West's terms?