When Nato leaders gather in Warsaw on Friday, they will face the most important decisions in the history of the alliance since the end of the Cold War.

The 28 members will be responding to one stark fact. In 2014, Russia seized Crimea from Ukraine, annexing the territory of a European country for the first time since 1945.

Afterwards, Russia invaded eastern Ukraine, starting a war that has claimed 9,000 lives and driven 1.7 million people from their homes. Nato’s eastern members, particularly those who border Russia, want assurances that they will never share the same fate.

Their concerns are still greater because Russia has conducted a series of military exercises designed to rehearse the invasion of neighbouring countries. Some of these drills have involved anything from 40,000 to 80,000 troops.