The first of 800 UK troops being sent to bolster Nato defences in the Baltic have arrived in Estonia.
The 120 soldiers from the 5th Battalion The Rifles will set up a UK headquarters in the country before the remaining troops arrive in April.
The defence secretary said they would deter "Russian aggression".
The UK is taking a leading role in Nato's "enhanced forward presence" operation, aimed at reinforcing the alliance's eastern border.
Tensions between Estonia and Russia increased as a result of the Ukraine conflict, which resulted in the annexation of Crimea by Russia in 2014.
Commander of the battalion Lt Col Mark Wilson said the British force would "integrate into the Estonian defence force, and bolster their capabilities".
He added that it was important for the UK to show solidarity with its Nato allies.
'Deter Russian aggression'
Defence Secretary Sir Michael Fallon told the BBC it was the biggest UK military deployment in Europe since the end of the Cold War and formed part of a long-term, open-ended commitment to deter Russia.
British Challenger 2 tanks, AS90 self-propelled guns, and armoured vehicles are en route to Estonia, having been loaded on to a ferry in Germany and will arrive next week.
He said deploying troops was necessary "because of the increased Russian aggression that we've seen and the need to reassure our allies on the eastern side of Nato".
But Sir Michael insisted it was a "defensive deployment" and was not "designed to provoke or escalate".
He added that Nato wanted to "deter any possible Russian aggression".
BBC defence correspondent Jonathan Beale said the troops say they are well prepared for a range of threats, and the UK government is aware that Russia might use misinformation, fake news and other provocations during their deployment.
The first wave of soldiers flew from RAF Brize Norton, in Oxfordshire, on Friday, and were met at Amari air base in Estonia by the country's defence minister, Margus Tsahkna.
Captain Gav Dunlop said that he felt the troops were trained for every eventuality.
He added: "There's obviously a credible threat just over the border but I think we're a credible deterrent so it just brings everything more to life."
The UK-led Estonia battlegroup is one of four Nato multinational deployments to eastern Europe, which were agreed to at the 2016 Warsaw summit.
Other Nato armies are sending forces to Latvia, Lithuania and Poland, where 150 UK personnel will also be sent as part of a rotating deployment.
Separately, the Royal Air Force has committed to providing Typhoon jets to bolster air defences in Romania for four months, as part of Nato's southern air policing mission.
Russia has described the positioning of Nato forces near its border as a threat.
In 2015 the Estonian government announced its intentions to build a fence along its eastern border with Russia to boost security.