This is The Guardian.

by Daniel Immervar.

The prospect of a Russian military invasion of Ukraine has become even more realistic in the past few hours,

following a televised address by President Putin.

Good morning from the Ukrainian capital Kiev, gunfire and explosions have been heard here and in the second city of Karkiv.

The Russian military claims to have taken control of Ukraine's black sea port of Heson in the south of the country.

Russia's war in Ukraine has involved many surprises.

The largest, however, is that it happened at all.

Last year, Russia was at peace and enmeshed in a complex global economy.

Would it really sever trade ties and threaten nuclear war just to expand its already vast territory?

Despite the many warnings, including from Vladimir Putin himself, the invasion still came as a shock.

But it wasn't a shock to the journalist Tim Marshall.

On the first page of his 2015 blockbuster book, Prisoners of Geography,

Marshall invited readers to contemplate Russia's topography.

A ring of mountains and ice surrounds it.

Its border with China is protected by mountain ranges, and it is separated from Iran and Turkey by the Caucasus.

Between Russia and Western Europe stand the Balkans, Carpathians and Alps which form another wall.

Or they nearly do.

To the north of those mountains, a flat corridor, the great European plane,

connects Russia to its well-armed Western neighbors via Ukraine and Poland.

On it, you can ride a bicycle from Paris to Moscow.

You can also drive a tank.

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