Britain used to boast the most powerful navy in the world. No more. That’s a serious problem for allies like the United States. Traditionally, Britain’s Royal Navy has been the U.S. Navy’s closest partner. The two have fought together against most every foe. So any weakening of the Royal Navy also erodes Washington’s naval power.Today, however, the Royal Navy is a shadow of its former self. Government budgeteers have repeatedly, and excessively, cut the numbers of its ships, planes and manpower. It can barely patrol the United Kingdom’s own waters, much less project British influence abroad. Though London officials now vow to reverse the decline, it might be too late. With morale plummeting, and its few remaining ships frequently malfunctioning at sea, the Royal Navy’s suffering might be terminal.The timing couldn’t be worse. The West is mobilizing to defeat Islamic State, deter an increasingly aggressive Russia and manage China’s meteoric rise as a world power. The British fleet’s collapse is an object lesson for cash-strapped governments struggling to balance competing budgetary needs in a seemingly ever more volatile world. Yes, navies are expensive. They require long-term planning, work and funding. In peacetime, the fleet’s benefit is often invisible, marked by the absence of overt conflict.Yet navies remain crucial to national defense. Patrolling international waters with sophisticated sensors and powerful, long-range weaponry, they can respond more quickly to crises and bring more firepower to bear than can air forces (which require nearby runways) and armies (which move slowly). Navies that die from neglect leave a void that rogue states, terrorists and criminals can quickly fill. It takes navies to keep an eye on vast ocean regions. Remove what was once the world’s leading fleet, and you create a virtual security vacuum.During World War Two, the British fleet was still dominant. On D-Day in 1944, it was able to send more than 900 British warships across the English Channel to escort the Allied troops who would liberate Europe from Nazi Germany.As recently as 1982, the Royal Navy could quickly muster no fewer than 115 ships — including two aircraft carriers carrying jet fighters, plus 23 destroyers and frigates — to retake the Falkland Islands from Argentina.Today, the British navy doesn’t even have jet fighters. It mothballed its last Harriers in 2010. It possesses just 89 ships. (By comparison, the U.S. Navy and Military Sealift Command, the Pentagon’s fleet of support ships, have roughly 400.)Britain’s fleet has declined amid steady defense budget cuts, from 4.1 percent of gross domestic product in 1988 to 2.6 percent in 2010. Reductions in 2010 sliced another 8 percent in real terms. As part of a defense review in 2015, London vowed to stop cutbacks on the fleet. But the damage has been done.On paper, the Royal Navy’s 89 ships include one helicopter carrier, six amphibious assault ships, six destroyers, 13 frigates, seven attack submarines and four ballistic-missile submarines. The rest are minesweepers, survey ships and other support vessels, many no larger than the U.S. Coast Guard’s small patrol ships.Only the six destroyers, 13 frigates and seven attack submarines can be considered true frontline vessels, with adequate sensors, weapons and protection to fight and survive in a battle with a sophisticated foe. The other ships require escort through dangerous waters.Roughly half the ships are in routine maintenance or training at any given time. Several others are committed to small standing patrols, which leaves just a handful of vessels to respond to emergencies.But that’s assuming there are enough sailors to operate the ships. The Royal Navy has shed people faster than ships. Britain had 39,000 sailors in 2000. It now has a little more than 29,000, at least 2,000 short of its authorized strength.Fleet planners tried to address the personnel shortage by sidelining two of its most powerful ships. This summer, for example, the Royal Navy placed the large Type 23 frigate HMS Lancaster in “extended readiness”: It was tied up pierside, its crew assigned to other vessels.Meanwhile, the new Type 45 destroyer HMS Dauntless suffered serious problems with generators and entered port for repairs that could last at least until 2019. As with Lancaster, the fleet dispersed Dauntless’ sailors to other vessels.With those vessels out of action, the Royal Navy’s real strength dropped from 26 fighting ships to an unprecedented modern low of 24.Last month, the new attack submarine HMS Ambush collided with a merchant vessel off Gibraltar. The sub suffered serious damage and limped back to Britain for repairs that could take months, if not longer.That accident reduced the Royal Navy’s undersea combat strength by nearly 15 percent. It was a stark reminder that Britain has almost no naval strength in reserve.As budget reductions cut deeper, the British fleet withdrew from much of the world. Before 2010, the Royal Navy played a leading role in efforts to curb piracy off the Somali coast. British frigates formed the core of various international task forces that patrolled the Indian Ocean and Red Sea. David Axe is the editor of War Is Boring and a regular contributor to the Daily Beast. He has written for Danger Room, Wired and Popular Science.