What is NATO and why is Finland joining?

Finland has become the 31st member of NATO in a historic strategic shift provoked by Vladimir Putin’s illegal invasion of Ukraine.

The Nordic country’s ascendancy to the western military alliance – marked by a flag raising ceremony at its Brussels headquarters today – means NATO’s border with Russia will roughly double in length, from the Baltic states to the Arctic.

Last year, the Kremlin’s all-out invasion upended Europe’s security landscape and prompted Finland – and its neighbour Sweden…