The British Army has begun its biggest commitment since Afghanistan.
It's taken command of the land element of NATO's new Very High Readiness Joint Task Force.
The 5,000-strong VJTF has been set up amid tensions between NATO and Russia. It's designed to move at only five days' notice.
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The force was set up in the face of a resurgent Russia, with most of its troops coming from Britain's 20th Armoured Infantry Brigade.
The Very High Readiness Joint Task Force was conceived at NATO's 2014 Wales Summit.
The VJTF would be a rapidly-deployable multinational land, air and sea formation.
It was to be the spearhead for an expanded 40,000-strong NATO Response Force.
The move followed Russia's Crimean annexation and suspected part in the Ukraine conflict.
NATO countries bordering Russia - like Poland and the Baltic States – felt threatened.
The alliance wanted the VJTF to reassure nervous allies.
Today the formation is a reality, with a land element ready to move at five days' notice.
Since the New Year, 60% of its 5,000 troops have been British.
The main contributor is 20th Armoured Infantry Brigade and its units are on high alert.
And the VJTF is not Britain's only offer of reassurance to Russia’s NATO neighbours this year.
5 RIFLES are to lead a multinational battle group in Estonia.
In April, 150 members of the Light Dragoons will deploy to Poland.
At least that’s NATO's plan - which will also need the support of US President-elect Donald Trump.