Leaders of the NATO military alliance on Friday meet for a landmark summit in Warsaw to show a united front to a resurgent Russia while addressing the fallout from Britain’s dramatic vote to quit the European Union.

The uncertainty over the UK’s future relationship with the US and EU comes as NATO prepares to green-light a historic revamp in the face of fears sparked by Russian intervention in Ukraine, even as the alliance seeks to tone down those concerns.

Secretary General Jens Stoltenberg on Friday said NATO will seek constructive dialogue with Russia and does not want a repeat of the Cold War.