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 HolodomorA famine in Ukraine between 1932 and 1933 that killed millions of people. Some describe it as a genocide, arguing that it was deliberately caused or exacerbated by the government of the USSR.. Now, once again, it is doomed to be the arena of conflict between greater powers, as Russian tanks prepare to roll across the border.

The conflict is already set to blight the lives of millions of Ukrainians. But it could still escalate further. NATO forces are lined up across the entire western Ukrainian border. As Russia advances through Ukraine, a single misstep could have Russian and American soldiers exchanging fire - or fighter jets colliding in mid-air. 

On Sunday, speaking from a meeting of world leaders in Munich, Boris Johnson warned that the conflict could be the "biggest war in Europe since 1945".

We have been down this road before. Between 1756 and 1763, Europe, North America and parts of Africa and Asia were engulfed in the Seven Years' War, which historians regard as the first-ever global conflict. More than a million people were killed.

Yet it all started with a simple border dispute in North America. France, which laid claim to most of the Mississippi basinA vast area covering most of the midwestern states of North America., wanted to build forts along its frontiers with the British colonies, a move they opposed. Tensions rose until 1754, when a hotheaded young British colonel called George Washington ambushed a small French force, provoking a fierce response.

As hostilities grew between these two great empires, Britain's ally PrussiaA former German state whose capital was in Berlin. In the late 19th Century it united its neighbours into modern Germany., seeing which way the wind was blowing, invaded the small state of SaxonyA small eighteenth-century German country that today is a state of Germany.. 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 PrincipIn 1914, Bosnia was occupied by Austria-Hungary, but many of its people were Serbs who wanted it to be part of Serbia. Princip was one of them, and he assassinated Franz Ferdinand to put pressure on Austria-Hungary to hand the territory over. assassinated Franz Ferdinand, the heir to the Austro-HungarianAustria-Hungary was the successor state to the Habsburg Empire. It spread across much of the Balkans and central and eastern Europe. It was dissolved after World War One. throne, it initially seemed like a minor crisis. 

But after Austria-Hungary responded by attacking SerbiaA country in south-eastern Europe that gained its independence from the Ottoman Empire in 1878., the small country's larger ally, Russia, declared war in turn. Germany supported Austria-Hungary. France supported Russia. Britain supported France and Russia. Over the following four years of global war, more than 68 million would die.

Some fear the same could now happen in Ukraine. A small incident could bring Russia and NATO into direct conflict. China might decide to back Russia or take advantage of the confusion to attack TaiwanAn island country in east Asia. China still claims to be the legitimate owner of Taiwan, while the Taiwanese government until recently claimed to be the legitimate government of the whole of China., and before long the world's great powers could be at each other's throats.

But others think this is unlikely. Throughout the Cold WarA period of diplomatic conflict between blocs led by the USA and the USSR that lasted from 1947 until 1991, when the USSR collapsed., both sides were careful never to get into direct confrontation. This is because they both have something that no one had in 1756, 1914 or 1939: nuclear weapons.

Russia has the largest stockpile of nuclear arms in history, with 6,257 in total. The USA is not far behind, at 5,550. China, France and the UK might seem comparatively puny with 350, 290 and 225 apiece, but that is still plenty of deterrence.

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 a world war, it has made conflict between the great powers more likely in the future.