London ? What would Napoleon or Lord Nelson make of this? Britain and France struck a historic defense deal Tuesday aimed at preserving military muscle in an age of austerity, pledging to deploy troops under a single command, share aircraft carriers and collaborate on once fiercely guarded nuclear programs.

French President Nicolas Sarkozy, left, and Britain’s Prime Minister David Cameron exchange documents during the signing of a treaty at an Anglo-French summit Tuesday at Lancaster House in London. Britain and France vowed to work hand-in-glove as their leaders ushered in an unprecedented era of defense cooperation by agreeing to create a joint force and share nuclear test facilities.

The often skeptical neighbors insist an era of unprecedented cooperation is a pragmatic fit for two cash-strapped allies, though many question if the storied enemies of the battles of Agincourt and Waterloo can truly overcome centuries of mutual suspicion.

Following talks in London, British Prime Minister David Cameron and French President Nicolas Sarkozy said Europe’s only nuclear-armed powers had set out plans to work closely for the next 50 years — forming a joint rapid reaction force, sharing warhead testing facilities and tackling together the threats from cyber warfare and the militarization of space.

Cameron told his Cabinet the deal would save hundreds of millions of dollars as Britain seeks to clear its national debts, while Sarkozy said he believed the pact will help protect all of Europe.

“This is a decision which is unprecedented and it shows a level of trust and confidence between our two nations that is unequaled in history,” Sarkozy told reporters, following a summit of key ministers from both countries.

Though British and French forces have fought together on fronts across the globe — including during both World Wars and the enemy occupations of France — the leaders insist the accord will signal the closest integration ever of their armed forces.

Under the deal, Britain and France will form a joint expeditionary force — a pool of at least 5,000 troops, including special forces, able to deploy under a commander from either nation.

They will in the future share their two aircraft carriers, when Britain’s new vessel comes into service in about a decade. Fighter jets will be able to land on carriers from either country, providing cover when one nation has its carrier in dock for maintenance.

To slash the hefty costs of maintaining their nuclear weapons, the nations will share specialist laboratories at the U.K. Atomic Weapons Establishment in Aldermaston, southern England, and a new center at Valduc, southeast of Paris.

British officials acknowledged the deal would involve closer cooperation than ever before on nuclear weapons, but insisted they would not divulge nuclear secrets.

“The result will make our citizens safer, more secure and better protected in the global age of uncertainty in which we now live,” Cameron said.

The U.S. said the deal would secure the standing of two major NATO powers. U.S. Secretary of State Hillary Clinton and Defense Secretary Robert Gates had both recently raised concern over the impact of cuts to European defense budgets.