On one side of the border between Russia and Ukraine, more than 100,000 of Moscow’s troops are massed. On the southern side, thousands of Ukrainian citizens are getting military training to repel what western intelligence says is a possible Russian attack.

This is one of the world’s most tense borders — where little more than 30 years ago no frontier existed. Ukraine and Russia were part of a vast Soviet Union, whose dissolution in 1991 has shaped the two countries’ relations ever since.

For many observers in Ukraine and the west, it is a history defined by the Kremlin’s desire to retain influence in what it calls its “near abroad”. For Russia’s President Vladimir Putin, it has been coloured by Nato’s expansion east, which he claims is a security threat to Russia.