Washington’s wing-man in Europe is losing clout on the world stage as its military forces shrink. (Matt Cardy/Getty Images)

— With Europe facing its shakiest security environment in a generation, Britain has slipped into a familiar role: Washington’s tough-talking wingman.

British leaders have led the rhetorical charge against the twin menaces of Russia and the Islamic State while browbeating reluctant European governments to wake up to the reality of a newly unstable continent.

But behind the flinty facade lies an unmistakable erosion in British power, one that has reduced Washington’s indispensable ally to a position that U.K. officials, military leaders and analysts acknowledge could leave the United States without a credible partner in taking on the greatest threats to global ­security.

“If the U.K. can’t do it, who else is the U.S. going to turn to in Europe?” said Gen. Richard Dannatt, a retired British army chief. “There’s no one else.”

Britain’s diminished military capacity is a product of years of stringent austerity policies that show no sign of easing.

Despite a too-close-to-call election in less than two months, neither major party has stepped up to shield the military from further cuts, reflecting a public weary of foreign interventions after campaigns in Afghanistan, Iraq and Libya that are widely seen as failures.

In the two conflicts that most directly imperil Europe today, Britain has been largely ­invisible.

Its contribution to the U.S.-led air campaign against the Islamic State is “strikingly modest,” according to a recent parliamentary report. And as Russia has eviscerated Ukraine over the past year, Britain has often been a bystander while others have tried to stanch the bleeding.

The British withdrawal from world affairs could soon accelerate, with budget cuts likely to take an even greater bite out of an already withered military.

A report issued this week by a respected British think tank, the Royal United Services Institute, found that Britain’s regular army could shrink to just 50,000 troops by 2019 – about half the number of the amount when the decade began and just a fraction of the figure from the height of the Cold War.

“The concern is that we’re going to fall from being a significant player to a bit-part player,” Dannatt said. “The U.K. isn’t of much use to the U.S. if we don’t have a worthwhile military force behind us. Anybody can talk tough. But if you don’t back it up, everyone just laughs at you.”

U.S. officials, normally unwilling to criticize their most stalwart ally, have been unusually open about their apprehension in recent days.

Last week, Gen. Ray Odierno, the U.S. Army chief of staff, said that he was “very concerned” about U.K. defense cuts and that they could result in British troops fighting within American ranks rather than alongside them in any future conflict.

On Tuesday, U.S. Ambassador to the United Nations Samantha Power told the BBC that she was alarmed by the “gap between the collective security needs that we all have and the resources we are bringing to bear.”

Although Power did not single out Britain for criticism, taking aim instead at Europe as a whole, she focused her comments on a NATO target for members to spend at least 2 percent of gross domestic product on defense.

The long-standing goal is met by only four members from the 28-nation alliance: the United States, Estonia, Greece and ­Britain.

But looming defense cuts — coupled with a recovering economy — mean that Britain is on the verge of missing the mark. Prime Minister David Cameron has resisted calls from within his own Conservative Party to pledge to continue spending at that level, despite his government’s efforts to compel other NATO members to hit the target during an alliance summit that Britain hosted just six months ago.

Britain is not the only NATO country to cut military spending even as the apparent threat to European security grows.

A study by the London-based European Leadership Network recently found that at least six countries are trimming their military budgets this year, despite a promise at last year’s summit to halt a decline that has been felt across much of the West while military spending in Russia and China surges ahead.

Ian Kearns, the network’s director, said the cuts reflect the reality of a recession-scarred continent constrained by economic needs at home. Politicians, he said, assume that there are few votes to be won in amping up defense spending
at the expense of domestic ­programs.

“And unless we come to the brink of direct conflict with Russia, that political calculus is not going to change,” he said.

By any measure, Britain remains a potent military force. It has the fifth- or sixth-largest defense budget in the world — rankings vary depending on the source — as well as a submarine-mounted nuclear arsenal that can canvass the globe and a permanent seat on the U.N. Security Council.

Responding to Odierno’s comments last week, Cameron asserted that Britain continues to be formidable.

“I know when I spend time with President Obama and others how much they appreciate the fact Britain is a strong and capable partner,” he said.

But Cameron’s military advisers have questioned how long that can continue given the trajectory of the cuts.

Writing in Britain’s Daily Telegraph on Tuesday, Gen. Peter Wall, who until September was Britain’s top military officer, asserted that Britain had assumed “a lower level of ambition for U.K. involvement in global security than ever before.”

With British defense spending down by 10 percent over the past five years, Britain lacks maritime patrols to hunt for submarines and faces a projected decade-long gap in carrier-strike capability.

The cuts, Wall wrote, have left British policymakers with few options for deterring Moscow.

“We can now see those consequences playing out in our reticence to counter Russian ­expansionism, and her interference in our airspace and offshore waters,” Wall wrote.

In the Middle East, as well, Britain has been reluctant to engage.

A report released last month by Parliament’s defense committee found that just 6 percent of coalition airstrikes against the Islamic State had been carried out by Britain, and that Australia, Germany, Spain and Italy had all been more actively engaged in the fight.

Britain’s Labor Party, which is running neck-and-neck with the Conservatives in pre-election polls, has vowed to halt Cameron’s “retreat from the world” but has been vague about how.

More troubling for the prime minister may be a revolt from members of his own party who think he has given the country’s defenses short shrift.

Among them is John Baron, a Tory backbencher and military veteran who led a push Thursday in Parliament to request that the government spend at least 2 percent of GDP on defense. Though only a small number of members voted, the measure received backing from a majority of those who did, including leading Conservative voices on national security.

“I’m not an interventionist. What I see is a need to talk softly and carry a big stick,” said Baron, who opposed the 2003 invasion of Iraq and the scuttled 2013 bombing campaign in Syria. “These are challenging times. Hostile nation states are rearming and becoming more aggressive. But what we seem to be doing is hollowing out our armed forces.”

Karla Adam contributed to this report.

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