China’s People’s Liberation Army has gradually increased numbers to at least 50,000, up from about 15,000 in June last year, Indian intelligence and military officials told the Wall Street Journal. It reported that numbers have mostly increased over the last few months.

India has also deployed tens of thousands of troops and heavy artillery has been sent to the region.

Much of the military build up has taken place in Eastern Ladakh, which overlaps Kashmir and Tibet, a key strategic area home to several critical rivers that supply vast amounts of waterflow to both China and India.

Much of the military build up has taken place in Eastern Ladakh, which overlaps Kashmir and Tibet—a key strategic area home to several critical rivers that supply vast amounts of waterflow to both China and India.

Asia’s two biggest superpowers have only been officially at war once, back in 1962. The war lasted a month and a day, but 722 Chinese soldiers were killed and 1,383 Indian troops were killed, according to the Institute of Peace and Conflict Studies. Thousands more on both sides were wounded and captured.

The hilly and jagged terrain of the Himalayas has been one of the main reasons why the two countries have not sparred more.

However, last June in the Galwan Valley, a Himalayan hill plateau, the two countries saw their bloodiest confrontation in decades, which saw 20 Indian soldiers and four Chinese soldiers killed.

Over the past year, India has repositioned some 50,000 troops along its Himalayan borders, while combat aircraft squadrons have been moved to northern bases and tanks have been redeployed.

Despite fighting three wars over the Kashmir region, Pakistan seems to be no longer Delhi’s biggest foreign priority. Around 200,000 troops are stationed at its border with China.

China and India, which are both nuclear-armed, have managed to avoid a significant confrontation in recent years but have never actually settled on an actual border—instead the two countries are separated by a vague 2,000 mile demarcation line. This is known as the Line of Actual Control.

India generally see its control extending to where Chinese forces withdrew at the end of the 1962 war, while China sees its control extending to what they held before the war.

In the east, China continues to claim Arunachal Pradesh—what India claims as its own state—and the Aksai Chin plateau in the west.

The two sides at times have kept to rules to stop conflicts from escalating, including prohibiting border troops from carrying guns. That rule was changed last year after the brutal clashes in the Galwan Valley, where troops bludgeoned each other with batons and clubs wrapped in barbed wire.

A spokesperson for the Chinese foreign ministry said that the situation at the border is stable and controllable.

“China believes that any arms race and infrastructure construction aimed at military control are not conducive to the maintenance of peace and tranquility in the border areas,” the spokesman told the Wall Street Journal.

Indian Defense Minister Rajnath Singh, accompanied by senior military officials including the Army Chief General M. M. Naravane, were in Ladakh at the end of last month to review military preparedness, a press statement said on the June 27.

Newsweek has contacted the Chinese and Indian foreign ministries for additional comment.