In one instance, the world blundered into war. In the other, the British and French stood by while Hitler violated the Versailles Treaty with impunity, encouraging the German dictator to grab further territory. Both scenarios are possible in the growing crisis in Crimea.

The real problem with what Vladimir Putin is doing in Ukraine is that he may miscalculate and start a chain of events that no one can foresee the outcome. Russia is not only conducting artillery drills on the Ukraine border, it is massing troops and issuing warnings about "protecting" the Russian speaking population of eastern Ukraine.

Russia said on Friday it reserved the right to protect compatriots in eastern Ukraine after clashes in the city of Donetsk in which one person was killed.

The Foreign Ministry said Thursday's violence in the industrial city, where many people speak Russian, showed the Ukrainian authorities had lost control.

A 22-year-old man was stabbed to death in the clashes between pro-Russian protesters and a crowd favouring European integration and denouncing Russian forces' seizure of Ukraine's southern region of Crimea.

"Russia is aware of its responsibility for the lives of compatriots and fellow citizens in Ukraine and reserves the right to take people under its protection," the Russian Foreign Ministry said in a statement.

Implying the pro-Russian protesters were not to blame, the ministry said peaceful protesters had been attacked by right-wing groups armed with pneumatic guns and batons who arrived from other parts of Ukraine.

Witnesses, however, said the pro-Russian demonstrators threw eggs, smoke bombs and other missiles and broke through a police cordon to beat their opponents with batons.

The death was the first reported in recent Ukrainian violence outside the capital, Kiev. Police detained four people accused of fomenting the clashes.

There it is in black and white - a ready made excuse for Putin to start carving up the rest of Ukraine after he annexes the Crimea. US Secretary of State John Kerry is in London meeting with Russiam FM Lavrov to try to head off Putin.

Mr Kerry was expected to warn Mr Lavrov that the disputed referendum being held in Crimea in two days and Russia's military intervention there could trigger concerted US and EU sanctions.

He has warned of "very serious steps" if Russia annexes the region.

Russia insisted at the UN Security Council on Thursday that it did "not want war" with Ukraine.

During an emergency meeting of the Council, Moscow's ambassador to the UN Vitaly Churkin defended the right of Crimea, which is predominantly ethnic Russian, to decide whether or not to join the Russian Federation.

Russia's military intervention followed the fall of Ukraine's pro-Moscow President Viktor Yanukovych on 22 February.

Mr Kerry had talks with UK Prime Minister David Cameron and Foreign Secretary William Hague before his meeting with the Russian foreign minister at the US ambassador's residence in central London.

Speaking after the meeting, Mr Hague said the US and Russian foreign ministers would find it hard to make progress.

"The fact that so far Russia hasn't taken any actual action to de-escalate tensions makes this a formidably difficult task today," he said.

Ahead of the talks, Mr Lavrov acknowledged the crisis was a "difficult situation" to be in. "Many events have happened and a lot of time has been lost," he told reporters.

Before his arrival in London, Mr Kerry urged movement on the Crimea referendum issue.

"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here [in Washington] with respect to the options that are available to us."

Does anyone think Kerry is up to this challenge? This is a guy who chided Vladimir Putin for using "19th and 20th century" tactics in the 21st century. In liberal fantasy land, he thinks that actually matters. Why not use 19th and 20th century tactics if they work? Also, Kerry knows that any "serious" action taken by the west won't be very effective. Lavrov knows he's basically bluffing.

Would Ukraine fight to keep Russians from "protecting" people in their eastern provinces? Would there be civil war in Ukraine - another ready made excuse for Putin to move militarily. I don't see how, no matter what the odds against them militarily, that the Ukraine government can sit idly by while Putin carves out a chunk of their homeland. They would lose whatever legitimacy they possess if they allowed the dismemberment of their country. As for the possibility of civil war, the Kiev government has demonstrated a lot of forebearance in trying to keep order in some eastern cities. They may have to push back more aggressively so that civil order can be maintained, thus risking the kind of conflict that would start the Russian tanks rolling over the border.

It's all up to Putin. How cautious does he want to be? Whatever happens in London between Kerry and Lavrov won't matter at all, as the Russian president ponders the near future.