British lawmakers vote in favor of a government motion to extend airstrikes against Islamic State into Syrian territory, after urging from Prime Minister David Cameron. (Reuters)

Britain on Wednesday night opted to join a U.S.-led bombing campaign against Islamic State targets in Syria, with Parliament endorsing a push by Prime Minister David Cameron after a raucous debate marked by accusations that revived the ghosts of Britain’s involvement in the Iraq war.

Hours later, the government announced that British airstrikes had begun in Syria.

The vote put an end to an awkward year in the close military alliance between the United States and Britain, during which the Britons joined the Americans in bombing the Islamic State in Iraq but drew a line at the Syrian border. The British government had reasoned last fall that — unlike in Iraq — the Syrian government had not invited Western intervention.

But after the Islamic State claimed responsibility for terrorist attacks last month that killed 130 people in Paris, Cameron vowed to expand his country’s military contribution to operations in both of the terrorist group’s main sanctuaries.

He made good on that pledge Wednesday night, receiving a decisive majority of support from the British House of Commons, with 397 lawmakers voting for airstrikes and 223 opposed.

What started out as simple protests in Syria has expanded to civil war and now an international crisis.

Dozens of members of the opposition Labour Party broke ranks and joined nearly all of Cameron’s ruling Conservatives in favor of a motion that describes the Islamic State as “a direct threat to the United Kingdom,” and that cites a U.N. resolution against the group as justification for attacks.

President Obama welcomed the Parliament vote and said the United States looks forward to having British forces flying with the coalition over Syria. “We ... will work to integrate them into our coalition air tasking orders as quickly as possible,” the president said in a statement.

Britain’s participation is unlikely to alter the military balance in Syria, though its military does bring precision-guided Brimstone missiles that are considered an asset for the U.S.-led alliance.

The more significant impact is likely to be political, signaling the reemergence of a vital U.S. ally that has seemed to retreat from global affairs in recent years following misadventures in Iraq and Afghanistan. Other countries, notably France, have been striking Islamic State targets in Syria for months.

“The question is a fundamental one, and that’s whether Britain is willing to stand with its allies,” said John Bew, a professor in the War Studies Department at King’s College London. “For Britain, this is a tentative step back on the world scene.”

U.S. Secretary of State John F. Kerry said Wednesday that he expects other NATO partners to decide soon how to step up their contributions to a military campaign in Syria that has been dominated by American air power.

Speaking at the end of a two-day meeting of NATO foreign ministers in Brussels, during which he pressed his 27 counterparts to contribute more, Kerry expressed gratitude to Cameron for asking Parliament to send warplanes over Syria. He also welcomed Germany’s decision to send as many as 1,200 troops in a noncombat role to support the 65-nation coalition fighting Islamic State militants.

A peace protester in London before a debate in Parliament on whether Britain should launch airstrikes against militants in Syria. Parliament ultimately decided in the affirmative. (Matt Dunham/AP)

Kerry said he expected a number of other countries to make decisions on how they will expand their efforts, in terms of military assistance and intelligence gathering or support in other ways, including medical facilities and flight refueling.

The resolution passed by Britain’s Parliament specifically rules out ground troops. But it ends what Cameron had ultimately argued was an arbitrary restriction that prevented British warplanes from striking Islamic State targets on one side of a border rendered meaningless as the terrorist group has conquered territory stretching across both Iraq and Syria.

In a passionate defense of his war plan from the green benches of Westminster Palace, Cameron argued that staying on the sidelines would be seen around the world — and especially in the Middle East — as a dangerous abdication of Britain's moral responsibility to act.

“Far from an attack on Islam, we are engaged in a defense of Islam. Far from the risk of radicalizing British Muslims by acting, failing to act would actually be to betray British Muslims and the wider religion of Islam in its very hour of need,” he said.

The vote badly divided Labour, with the party’s pacifism-inclined leader, Jeremy Corbyn, vehemently opposed. Corbyn might have been able to block the government’s proposal had he insisted that members of his party join him in voting against it.But faced with a rebellion — and threats of resignation — from some of the party’s most senior members, he instead opted Monday to allow Labour politicians “a free vote.”

The choice likely guaranteed Cameron a victory. But Cameron seemed to hurt his own cause Tuesday night when he urged wavering members of his party not to side with “Jeremy Corbyn and a bunch of terrorist sympathizers.”

Repeatedly pressed to apologize Wednesday, Cameron declined while saying he respected those with different views. “I respect that fact that we’re all discussing how to fight terrorism,” he said, “not whether to fight terrorism.”

Corbyn on Wednesday dismissed the idea that airstrikes can be effective in ending the Syrian war, and he cited British involvement in Iraq as evidence that military campaigns spawn more violence and anti-Western sentiment.

Britain’s participation in the 2003 invasion of Iraq was guided by a Labour prime minister, Tony Blair — a man whom Corbyn has said could be subject to war-crimes charges.

Amid persistent heckling from Conservatives, Corbyn said airstrikes in Syria would lead to “unintended consequences,” including more civilian casualties as well as a heightened motivation for extremists to attack Britain.

He urged members of Parliament to “step back and vote against yet another ill-fated twist in the never-ending war on terror.” Britain, he said, should focus on peace talks, rather than a bombing campaign that would lead to “the deaths of innocents.”

But one of the most compelling speeches of the day in favor of bombing Syria came from Corbyn’s point person on foreign policy, Hilary Benn, who delivered a rousing call for Britain to stand up to “fascists.”

“These people hold us, and our values, in contempt,” he said.

The speech was greeted by applause from both sides of the chamber as Corbyn wore a stern look.

Britain’s entrance into the Syrian air campaign adds to skies already crowded with warplanes.

The United States has conducted the vast majority of allied bombing runs in Syria, but France has escalated its contributions in the weeks since the Paris attacks. Saudi Arabia, Jordan, the United Arab Emirates, Bahrain and Qatar also have taken part, though their participation has fallen off substantially.

Syrian warplanes, meanwhile, lately have been joined by Russian jets, which have aggressively targeted groups opposed to the government of President Bashar al-Assad.

Read more:

Obama defends his Islamic State strategy in Syria

NATO faces new Mideast crisis after downing of Russian jet by Turkey

The difficult path to end Syria’s war

Morello reported from Brussels. Karla Adam contributed to this report from London.