On Thursday, British Prime Minister Boris Johnson will caution Russian President Vladimir Putin against invading Ukraine, using a visit to NATO’s headquarters to emphasize European cooperation in the face of Russian aggression.

Russia, which has more than 100,000 troops stationed on Ukraine’s border, denies any intention of invading. The United States, the United Kingdom, the European Union, and NATO are concerned that Putin is planning to seize Ukraine.

As Johnson deals with a domestic fight in Downing Street about lockdown parties, he will strive to present himself on Thursday as the European leader of efforts to avert what may be Europe’s worst security crisis since the Cold War.

“The UK remains unshakable in its commitment to European security,” said Johnson, a key figure in the 2016 Brexit referendum campaign that resulted in Britain exiting the EU.

“As an alliance, we must draw lines in the snow and be clear about whose ideals we will not compromise,” Johnson added. “This encompasses the protection of every NATO ally, as well as the freedom of every European democracy to seek NATO membership.”

Foreign Secretary Liz Truss will be in Moscow as Johnson travels NATO and then Poland. Truss will pay a visit to Moscow State University and meet with Sergei Lavrov, the world’s most experienced foreign minister.

Downing Street claimed Britain was spearheading international efforts to settle the problem, despite the fact that French President Emmanuel Macron met with Putin for hours in the Kremlin on Monday.

Putin, who claims that Moscow’s worries about NATO enlargement have been disregarded for three decades, has requested guarantees that there would be no missile installations near its borders and that the military alliance will not be expanded further.

NATO is a defense alliance, according to the West, but Putin has frequently denied this, citing conflicts in the Balkans, Iraq, Syria, and Afghanistan.

Putin warned Macron during a press conference this week in the Kremlin that if Ukraine ever joins NATO, Russia and the NATO alliance may go to war.

“If Ukraine joins NATO and seeks to reclaim Crimea through military methods, European nations will be drawn into a military clash with Russia,” Putin told reporters.

According to Defence Secretary Ben Wallace, no one would benefit from an aggressive invasion of a sovereign state.

“What we’re actually all trying to do, whether you’re in NATO or not, is safeguard countries’ sovereign right to pick their security alliances,” Wallace explained.