Few parts of the world have seen as much bloodshed as Ukraine. In the 20th Century, millions of its people perished in successive world wars and the Holodomor. Now, once again, it is doomed to be the arena of conflict, as Russian tanks roll across the border.

And the conflict could still escalate further. As Russia advances through Ukraine, a single misstep could have Russian and American soldiers exchanging fire, or fighter jets colliding in mid-air. Such an event could even spark a world war.

We have been down this road before. Between 1756 and 1763, the world was engulfed in the Seven Years' War, which historians regard as the first ever global conflict.

Yet it all started with a border dispute in North America. France wanted to build forts along its frontiers with the British colonies, a move they opposed.

As hostilities grew between these two great empires, Britain's ally Prussia, seeing which way the wind was blowing, invaded the small state of Saxony. French allies Austria and Spain, then Russia, responded with force, and suddenly all the great powers were at war.

The same happened again in 1914. When Gavrilo Princip assassinated Franz Ferdinand, the heir to the Austro-Hungarian throne, it initially seemed like a minor crisis. 

But after Austria-Hungary responded by attacking Serbia, Russia declared war. Germany, France and Britain followed.

Some fear the same could now happen in Ukraine. A small incident could bring Russia and Nato into direct conflict.

But others think this is unlikely. Throughout the Cold War, although Russia and America were enemies, both sides were careful never to let their forces get into a direct confrontation. This is because they both have something no-one had in 1756, 1914 or 1939: nuclear weapons.

Yes: History teaches us that world wars usually spiral from small incidents. And with Nato and Russian forces packed so tightly, there is ample opportunity for tensions to boil over.

No: The Ukrainian war is unlikely to last very long. Vladimir Putin wants to extract concessions and get out quickly. There will be little time for an international incident that could spark a wider war.

Or: Russia has proved it is willing to defy US power. China will be watching with interest. Even if this crisis does not spark world war, it has made conflict between the great powers more likely in the future.