Hundreds of UK troops are to be sent to Poland and Estonia as part of the Nato response to concerns over Russia.
A 500-strong battalion will be deployed to Estonia and 150 troops will go to Poland to "reassure" these countries, Defence Secretary Michael Fallon said.
It is part of a Nato commitment made after Russia's annexation of Crimea from Ukraine in 2014 to have four new battalions on its eastern flank.
Mr Fallon said it aims to "deter Russia from any further aggression".