West's options: Russia's Ukraine challenge
Russia has taken control of Crimea, and pro-Russian militants are asserting control in eastern Ukraine.
Western countries have responded with some sanctions and tough rhetoric. What else could they do to contain Russia?
Stick to diplomacy
The obvious route for increasing diplomatic pressure - the UN Security Council - is blocked because Russia is a permanent member and can veto any proposed action. Other organs of the UN, such as the General Assembly, could pass non-binding resolutions, but Russia could simply ignore them.
Where it has been able to, the West has made tentative moves towards isolating Russia, cancelling a G8 meeting earlier this year and suspending Moscow's membership. But Russia cannot be so easily kicked out of the Security Council, the consensus-based Organisation for Security and Co-operation in Europe - custodian of the Helsinki Final Act that does not permit forcible changes of borders - or the World Trade Organization Moscow waited so long to join. Countries are then left with the possibility of taking bilateral action through visa bans and other restrictions.
There is always a question mark over punitive measures. Are they a help or a hindrance? As a broad principle, Western nations publicly preach the benefits of engagement over isolation and regard the cold-shoulder as an extreme measure.
But engagement has failed to check the Russian advance - or force an about-turn in Moscow. Russia acted fast in annexing Crimea, and the interim government in Kiev now fears that Moscow plans to swallow up more of Ukraine. Russia could then move on to other countries with Russian-dominated areas, such as Moldova's Trans-Dniester region. The West's tacit concession of the Crimea issue has done nothing to strengthen its diplomatic hand.
The military option
Additional aircraft have already been deployed to patrol the Baltic air-space and US jets are carrying out exercises in Poland. Units of paratroopers are being despatched to Poland, Lithuania, Latvia and Estonia.
This will establish a pattern of small-scale exercises that are likely to continue over the course of this year. They are probably adequate for their intended purpose of reassuring anxious Nato allies in the region - but not enough as a deterrence.
It is almost inconceivable that the current Ukraine crisis will lead to a full-scale military confrontation between Nato and Russia. Nato is not going to deploy troops in Ukraine. If Russia were to move its troops overtly into eastern Ukraine the response would likely be severe economic sanctions - not military action.
However, any moves by Russia to threaten or destabilise the Baltic republics could lead to a potential military confrontation.
In the longer term, wholesale increases in Nato defence spending are unlikely and the alliance will have to look again at how best to defend its anxious members who live close to Russia's borders.
Expand economic measures
There is likely to be a split between the EU and US on the possibility of widening economic sanctions. European firms have much closer ties with Russia, and the eurozone's economic recovery remains fragile.
But the US does far less business with Russia, and could go much further that Europe. It has already imposed sanctions on Bank Rossiya and imposed asset freezes and visa bans on Russian politicians, businesses and military figures. Experts believe the first step would be widening that sanctions list.
New sanctions could target a wider selection of state companies, including those beyond President Putin's inner circle. Denying particular technology used in agriculture and metal processing could also boost the effect of sanctions.
But the West seems unwilling to tackle Russia's energy sector. Europe's reliance on Russian gas makes it particularly vulnerable to Russian reprisals. On this, the US would be alone.
The West may believe that its moderate sanctions, rhetoric and military posturing will deter Russia - simply watch and wait and Russia will come back into line.
However, some Kremlinologists suggest President Vladimir Putin is trying to forge a greater Russia based on an anti-Western, semi-mystical philosophy. Others that his aim is to bring order to a chaotic part of the world.
If the "do nothing" gamble failed, the knock-on effects could be felt around the world. Leaders of less illustrious states than Russia could feel emboldened to seize a piece of land they have always hankered after; Crimea-style secession rebellions could mushroom, threatening stability and economic development.