WASHINGTON — The Senate on Wednesday ratified the accession of Finland and Sweden into NATO, approving one of the most significant expansions of the military alliance in decades as the West fortifies Europe against Russian aggression.

Senators voted 95-1 to endorse membership for Finland and Sweden, more than the required two-thirds majority of 67 votes. Most of NATO’s 30 members have already signed off on the accession protocols for Finland and Sweden, ushering in a historic end to the many years of neutrality and military nonalignment for the Nordic nations.

The U.S. had hoped to be the first to approve the addition, said Sen. Jeanne Shaheen, D-N.H., but it was the 23rd.

"Today – at a moment when democracy in Europe is under attack, as belligerent autocrats like [Russian President Vladimir] Putin clamor for European dominance – the U.S. Senate is voting in overwhelming bipartisan fashion to approve Finland and Sweden’s accession to the NATO alliance," said Senate Majority Leader Chuck Schumer, D-N.Y. "This is important substantively and as a signal to Russia: They cannot intimidate America or Europe."

Schumer invited ambassadors and other diplomats from the two nations to the Senate gallery to witness the debate and vote. Minority Leader Mitch McConnell, R-Ky., described the ratification as a “slam dunk for national security” and admonished those who opposed it.

“There's just no question that admitting these robust democratic countries with modern economies and capable, interoperable militaries will only strengthen the most successful military alliance in human history,” McConnell said on the Senate floor. “Their accession will make NATO stronger, and America more secure.”

Senators on Wednesday also approved an amendment proposed by Sen. Dan Sullivan, R-Alaska, that reinforced NATO members' defense spending levels. The chamber overwhelmingly shot down an amendment proposed by Sen. Rand Paul, R-Ky., ensuring that NATO's guarantee to defend its members does not supersede Congress's authority to declare war.

The legislatures of all NATO allies need to agree to admit Finland and Sweden before the countries could be protected by the alliance’s mutual defense clause, which states an attack on one member is an attack against all members. The process is expected to take a year. In the meantime, Finland and Sweden can attend NATO meetings and have greater access to intelligence.

The two countries applied for membership in May, less than three months after Russian forces stormed Ukraine in late February. The invasion shattered the Nordic region’s longstanding sense of stability and spurred a dramatic surge of public support for joining an alliance formed 73 years ago to counter the growing power of the former Soviet Union.

Putin partly blamed Ukraine’s ambitions to enter NATO for provoking his invasion and Russia has repeatedly warned Finland and Sweden against joining the alliance.

The U.S. and NATO have long partnered with Finland and Sweden on defense and troops from both countries have contributed to missions in Kosovo, Afghanistan and Iraq. But their official inclusion in the alliance is expected to significantly alter the security landscape in Northern Europe, where melting Arctic ice is opening new opportunities for transit and exploration and increasing naval and air activity, experts have said.

With the addition of Finland, NATO’s border with Russia will extend by 830 miles.

Finland is an outlier in Europe for holding onto its ground forces after the end of the Cold War and can quickly muster some 300,000 troops within 30 days of a crisis, said William Alberque, director of strategy, technology and arms control at the International Institute for Strategic Studies, a research think tank in London.

The country also has the third largest artillery force in Europe, behind Russia and Ukraine, and is building up one of the largest fleets of F-35 fighter jets on the Continent, he said.

The Finns regularly train for a Russian attack, a lasting effect of the four-month attempt by Soviet troops to seize Finnish territory shortly after the outbreak of World War II, Alberque said. Finland was outmanned and outgunned in the Winter War but repelled the massive invasion and inflicted mass casualties on the Red Army.

“It’s funny when people say, ‘No one could’ve imagined this scenario,’” Alberque said of Russia’s invasion of Ukraine. “Well, the Finns thought about it all the time.”

Army Gen. Christopher Cavoli, NATO’s supreme allied commander and leader of U.S. European Command, attested to Finland’s military readiness during his confirmation hearing in May, telling senators he was “very impressed” with patrols along the Russian border.

“The Finns are absolutely expert in defending that border,” Cavoli said.

Alberque said Finnish people share an affinity for the military that can only be compared to the close relationship between Americans and their armed services. Opinion polls show Finland has the highest concentration of citizens in Europe who are willing to fight and die for their country, he said.

That commitment is reflected in Finland’s defense spending, which meets NATO’s benchmark of 2% of a country’s gross domestic product. Sweden has pledged to match that target by 2028.

“Their participation in NATO would reduce the burden on the United States and the whole military alliance,” said Sen. Bob Menendez, D-N.J., chairman of the Senate Foreign Relations Committee.

The U.S. is by far the largest defense spender in the alliance, accounting for 51% of NATO’s combined GDP and 69% of combined defense expenditure, according to a March report by NATO Secretary General Jens Stoltenberg. Sen. Josh Hawley, R-Mo., said he voted against Finland and Sweden membership because the U.S. needs to divert those dollars to challenging a rising China.

Sweden offers NATO one of the most robust defense industries in Europe with an ability to manufacture its own fighter jets, armored vehicles and artillery, Alberque said. It also has a powerful Navy with five submarines and dozens of smaller ships. The Swedish army is small but growing after a military exercise held in the aftermath of Russia’s 2014 annexation of Crimea showed Sweden would hold out only two or three days against a Russian invasion, he said.

The country’s naval capabilities alone will give the alliance dominance over the Baltic Sea, a key waterway for Russian vessels traveling to the Atlantic Ocean and home to the Russian exclave of Kaliningrad, headquarters of Russia’s Baltic fleet, Cavoli said in May. Sweden’s strategically located Gotland, an island that the country is outfitting with more barracks for troops, sits in the center of the sea and is less than 200 miles from Kaliningrad.

The combined firepower and geographic footprint of Finland and Sweden will force Russia to spread its forces thinner, Alberque said. It also will need to bulk up defenses around St. Petersburg and rush more troops to the crucial Kola Peninsula, a landmass 110 miles east of the Finland-Russia border where Russia bases its nuclear submarine fleet and Arctic navy.

“You’ve got Swedish and Finnish air radars, anti-aircraft, anti-ship and land attack capabilities. You’ve got a massive Finnish army within days of a crisis initiating. You’ve got Poland with U.S. forces, Lithuania with German forces, Estonia with more and more NATO forces,” Alberque said. “It’s looking more and more like a nightmare for Russia.”