This week European nations adopted unprecedented sanctions on Russia. These are not designed to punish the Russian people. They are a message and a warning to Russia’s leaders.

We have not embarked on this course lightly. For twenty years Britain and our allies in Europe and Nato have gradually built a normal relationship with Russia. Russia joined the G8 and the World Trade Organisation, we created the Nato-Russia Council, and we have worked together on the United Nations Security Council from Yemen to Somalia. We have all gained from this cooperation.

But Russia’s actions in Ukraine hark back to a wholly different era. The illegal annexation of Crimea is an outrageous land grab, and the referendum that preceded it was a mockery of democracy. Russia has invaded a fellow European nation, and used force to change its borders.

This is the most serious risk to European security we have seen so far in the 21st century. Vital principles are at stake, including the territorial integrity of European nations and the right of democratic countries to choose their own future.

All nations, including Russia, depend on a rules-based international system. For those rules to remain credible there must be costs attached to breaking international agreements. If we do not defend these principles in Ukraine, they will be threatened elsewhere in Europe and the world.