The elite 2,400-person unit includes only about 40 women.

Three of them are Hikari Maruyama, Runa Kurosawa and Sawaka Nakano. Living alongside a close-knit group of other female service members aboard the JS Osumi, a Japanese Maritime Self-Defence Force tank landing ship deployed for exercises in the East China Sea, in November they supported beach assault drills in Japan’s vulnerable southwest island chain.

Although they and their fellow marines are expected to lead the way on the front lines, their unit – and Japan’s military – lag far behind in gender diversity, a problem that risks turning into a crisis as the country’s greying population shrinks while threats from China, Russia and North Korea grow.

“Women are crucial to ensuring a stable supply of suitable recruits,” Shingo Nashinoki, then-commander of the ARDB force, said on an uninhabited island in the Okinawan chain, where a small all-male ARDB contingent practiced helicopter attacks.

Although the number of female Japanese soldiers has doubled during the past decade, it is still far behind Tokyo’s ally, the United States.

Women make up only 8.7 percent of the 230,000 strong Japanese Self-Defence Forces (JSDF), half the rate of the US military, and only 1.6 percent of the ARDB, which was activated in 2018. By comparison, almost one in 10 US Marines are women.

“The ARDB has a reputation for being physically, mentally and technically demanding, and I think that a lot of women worry whether they could handle that,” said Staff Sergeant Maruyama, 38.

Aboard the Osumi, women are uncommon. Only men participated in a fitness training session on the flight deck. Maruyama and Corporal Kurosawa, 20, instead stretched in the ship’s small gym while male colleagues around them lifted weights.

The JSDF’s efforts to present itself as a more female-friendly force have been undermined in recent months by high-profile sexual harassment cases.

In October, Minister of Defence Minoru Kihara had to apologise after a Japanese sailor was forced to meet a superior accused of sexually harassing her. In December, a Japanese court found three male soldiers guilty of sexually assaulting a female comrade.

“It leaves me a bit speechless. It’s important to be clear to every person what harassment is and to continue educating people,” said Captain Nakano, 42, aboard a ship that was not designed to accommodate a mixed-gender crew when it was commissioned 20 years ago, before she joined. She would like to see more roles opened to women, she added.