Some commentators forecast a return to the rivalries of Cold War - but with a more complex and darker twist that will test international law and global politics in ways that rarely happened in the US-Soviet standoff.
In seizing the Ukrainian peninsula, Putin argued Russia had the right to intervene to protect ethnic Russians there and recover land handed from Russia to Ukraine during the time of the Soviet Union. Yet Ukraine's own borders were legally endorsed by Russia itself in 1994. For many Europeans, Putin has given a terrifying justification for land-grabs in Moldova, Belarus and the Baltic states where there are Russian minorities. But by assaulting international law, his move also has repercussions beyond Europe, setting a precedent for any powerful country that wants to bully a weaker one.
"Russia's recent conduct is often framed narrowly as the start of a new cold war with America. In fact it poses a broader threat to countries everywhere because Mr Putin has driven a tank over the existing world order," the Economist commented. "Mr Putin's new order, in short, is built on revanchism, a reckless disdain for the truth and the twisting of the law to mean whatever it suits those in power. That makes it no order at all."
The crisis has sparked soul-searching in Brussels, which seems to have badly misjudged Putin and the risks arising from the downfall of his Ukrainian ally, Viktor Yanukovych.
After three weeks of firefighting aimed at preventing the crisis from worsening, the upcoming summits offer leaders a chance to shape the West's longer strategy, say observers.
"In the Crimea, Russia scored appreciable gains in comparison to the diplomatic cost. It has to understand that the cost-benefit outcome will be less favourable for annexing other Ukrainian provinces," Cyrille Bret, a political scientist at France's elite Sciences-Po school, told the Herald by email.
Leaders will have to mull whether to increase military spending, which has been on a declining path since the end of the Cold War, and revive the forward-defence posture in frontline Nato states, as they successfully did to ward off the Soviet threat. At the risk of inflaming Putin, they will have to decide about whether to provide Ukraine with military assistance and, eventually, renew the prospect of Nato membership. The alliance offered Ukraine this in 2008, but it was turned down in 2010 by Yanukovych.
Western leaders will also have to look at increasing the political price for Russia, excluding it from big international gatherings and organisations and diplomatic processes, such as the conflict in Syria and Iran's nuclear programme, to discourage it from further seizures.
Obama is to meet in The Hague with President Xi Jinping of China, whose country abstained in a UN Security Council vote on Crimea. Britain, Canada, France, Germany, Italy, Japan and the United States may formally suspend Russia from the G7 gathering, according to a reliable source in Brussels.
"What will be clear for the entire world to see is that Russia is increasingly isolated and the United States is leading the international community in supporting the government of Ukraine and the people of Ukraine and imposing costs on Russia," said US National Security Adviser Susan Rice.
The EU, for its part, is talking of speeding up talks to conclude political and economic pacts with other post-Soviet republics such as Moldova and Georgia - a process that is grievous to Putin, who laments the loss of Russian influence since the breakup of the Soviet Union.
Another strategic path will be how to shore up Ukraine to prevent it from imploding after revolution.
"If you don't support Ukraine politically and economically, then you are basically saying to the Russians you don't really care," Bobo Lo, a former Australian diplomat in Moscow and an associate fellow at the Chatham House think-tank in London, told the Herald.
The task is huge, said Lo. The country is bankrupt, rife with corruption and reeling from the loss of Crimea.
