‘This is our 1937 moment,’ the U.K.’s new army chief says of the war.

The British Army must act rapidly to prevent any further military action by President Vladimir V. Putin of Russia beyond the borders of Ukraine, the force’s new head said on Tuesday, adding that Britain must be ready to defend NATO territory if necessary.

“This is our 1937 moment,” the military leader, Gen. Patrick Sanders, said, making reference to Western Europe’s failure to confront Nazi Germany’s territorial expansion in the years preceding World War II. “We are not at war, but we must act rapidly so that we aren’t drawn into one through a failure to contain territorial expansion,” he said.

The British army chief’s comments, his first speech since taking up his post, came as Prime Minister Boris Johnson of Britain prepares to meet Western leaders at a NATO summit in Madrid, where they are expected to agree on the alliance’s biggest overhaul since the end of the Cold War.

Mr. Johnson is expected to announce a rise in military spending this week, building on recent increases, and Britain’s defense secretary, Ben Wallace, is also expected to call for increased military spending on Tuesday.

General Sanders said that Britain would lead by example to confront the threat posed by Russia and that his own army was mobilizing to prevent, rather than provoke, war.

“Russia will be an even greater threat to European security after Ukraine than it was before,” he said.

General Sanders said that deterring Russia would involve a higher degree of readiness and training. He also said that any move to further reduce Britain’s troop numbers would be “perverse” given the situation in Ukraine, but that he would be open to a possible restructuring of the army.