The NATO summit in Madrid could see fissures and debate beneath a semblance of unity.

BRUSSELS — After the collapse of the Soviet Union, some asked whether NATO had any real reason to exist. But Russia’s invasion of Ukraine gave new urgency to NATO as a defensive alliance aimed at deterring a bellicose Moscow.

Now, at its annual summit meeting this week in Madrid, leaders of the 30 members of NATO are set to agree on the most significant overhaul of the alliance’s defenses since the Cold War.

The war in Ukraine has radically altered the security calculus in Europe, presenting the alliance with one of the greatest challenges in decades and alarming jittery allies among NATO countries in East and Central Europe and the Baltics, who are all too familiar with Russian subjugation.

Among the measures expected in Madrid will be a large increase in the number of troops assigned to defend NATO’s eastern flank, closest to Russia and Belarus. There will also be a major commitment to position heavy military equipment there, like tanks and artillery, that would bolster an allied response to any Russian threat or aggression.

The Russian invasion has laid bare President Vladimir V. Putin’s apparent intention to wind back the clock more than 30 years, and to establish a broad, Russian-dominated security zone resembling the power Moscow wielded in Soviet days.

But while Russia’s invasion has helped foster unity in the face of Russian aggression, fissures remain, along with internal debates within the alliance on how long the war will take, how it will end and its mounting costs to NATO and European allies.

There will also be discussion this week on how to convince Recep Tayyip Erdogan, the president of Turkey, to remove his effective veto over the applications of Sweden and Finland to join NATO, a matter that inevitably will have to involve President Biden.

By seeking to join NATO, Sweden and Finland have abandoned decades of neutrality and nonalignment, underlining how the war has helped spur the potential expansion of an alliance Mr. Putin had been seeking to tame.

In Madrid, the alliance will also approve its first updated mission statement in 12 years, portraying a world of new perils, with threats not just from Russia but also China, an American priority, and from new forms of warfare ranging from cyber and artificial intelligence to disinformation and restrictions on energy, food and rare minerals.