Japan, which at the start of World War II was the proprietor of the world’s mightiest aircraft carrier fleet, is getting back into the naval aviation game after decades. Under pressure to counter a growing Chinese navy and air force, the island country is two “helicopter destroyers” into full-fledged aircraft carriers, built to carry F-35B Joint Strike Fighters.

The two carriers could even see U.S. jets operate off them before Japanese planes, a sign of the very close alliance between Washington and Tokyo.

In December 1941, Japan had the most powerful force of aircraft carriers in the world. The Imperial Japanese Navy had 10 such vessels—six fleet carriers and four light carriers—world beating aircraft aboard, and an elite corps of naval aviators. Japanese aircraft carriers rampaged across half the world, from to Pearl Harbor.

Four years later, hammered by the combined might of the Allies, Japan had lost 21 aircraft carriers and thousands of pilots. The militaristic government was dissolved and the country committed to pacifism, swearing off war as an instrument of national policy. It relied on a Self-Defense Force for protection and banned carriers and other offensive weapons of war.

Now, 75 years later, Japan is officially planning to equip its naval forces with aircraft carriers again. The country will convert the warships Izumo and Kaga to launch and recover F-35B Joint Strike Fighters and will —the vertical takeoff version of the Joint Strike Fighter used by the U.S. Marines—to operate from them.

So after decades of defending itself without carriers, how did this happen?

The one-word answer: China. That nation’s roaring 30-year economic boom also grew its defense budget to become second-largest in the world. In 1989, China spent just $19 billion on defense. In 2018, military spending, pegged to just over two percent GDP, and is still growing. This increase in defense spending has dramatically modernized China’s armed forces, particularly its air and sea forces.

China’s newfound military power has also emboldened Beijing. China’s relations with neighbor and traditional adversary Japan , and Japan has seen a rapid increase in Chinese air and naval activity in the Sea of Japan and East China Sea. Tensions between China and other Asian nations have escalated for years.

In December 2017, People’s Liberation Army Air Force fighters were spotted for the first time in the Sea of Japan, and in April 2018 China’s first aircraft carrier, Liaoning, . Japanese fighters in 2018 in response to Chinese Navy and Air Force aircraft near Japanese waters, up from 500 the year before.

Most of the Japan Air Self-Defense Force’s scrambles take place in the East China Sea, off the southern edge of Japan near Okinawa. Only one airbase on the island, Naha, is close enough to quickly respond. (Naha is also a combined civilian and military airport and regularly juggles commercial air and Japanese military traffic.) Naha is home to forty F-15J Eagle fighter jets but just one, vulnerable airport runway.

In other words, just one Japanese airbase is holding the line against Chinese air and naval power.

If a shooting war breaks out, China’s arsenal of missiles, including the , will shut Naha down in minutes. The Japanese military would like more airbases in the region but that’s unlikely, as the densely populated—and hilly—country simply doesn’t have any room to spare for sprawling military bases and their noisy fighter planes.

Enter, the solution: put the aircraft on ships that move and don’t present a fixed target. Izumo and Kaga, built with full-length flight decks, hangars, and elevators, could probably carry a dozen F-35Bs each. Both ships were built to carry anti-submarine helicopters but also with the F-35B in mind. Each ship will need expanded storage for aviation fuel and aircraft weapons, a seagoing version of the , and a thermoresistant coating to protect the flight deck from the punishing heat of a F-35B’s exhaust nozzle. According to the , Japanese shipyards will finish modifications to Izumo in 2020 and Kaga in 2022.

Japan’s aircraft carriers will be ready for action before the F-35Bs are delivered, so U.S. Marine Corps F-35Bs . Launching and recovering the American jets will be valuable experience for the Maritime Self Defense Force, getting ship crews reacquainted with the hard and dangerous work of naval aviation. The Maritime Self Defense Force borrows heavily from U.S. Navy procedures and tradition, and U.S. and Japanese forces conduct joint exercises on a near-weekly basis, so the team-up won’t be as difficult as one might expect. USMC F-35Bs are also on HMS Queen Elizabeth, the U.K.’s new aircraft carrier, in 2021.

But wait a minute—how did Japan get around the ban on aircraft carriers?

Simple: Japan banned aircraft carriers as “offensive weapons of war” but defensive weapons have always been A-OK. The new carriers are meant as defensive weapons. Further distancing the ships from Japan’s aggressive past, Tokyo has engaged in semantic sidestepping, labeling the ships “multi-purpose operation destroyers.”

Jet fighter-carrying destroyers, got it? Good.