Russia is getting ready for war games with 300,000 troops — but the size isn't the only 'unprecedented' thing about it

.The latest iteration of Russia's Vostok military exercise, set for mid-September, has attracted attention for its massive scale.

.But just as striking a feature may be the inclusion of China.

.Russia has long seen China as a rival, but tensions with the West appear to have triggered a shift by Moscow.

Russia's armed forces are gearing up for Vostok-18, or East-18, a massive military exercise in the country's far east from September 11 to September 15.

Defense Minister Sergei Shoigu said this week that about 300,000 troops and 1,000 aircraft would participate, using all of the training ranges in the country's central and eastern military districts. Russia's Pacific and Northern fleets and its airborne forces are also expected to join.

Shoigu said this year's iteration of the Vostok exercise would be "unprecedented in scale, both in terms of area of operations and numbers of military command structure, troops, and forces involved."

But the size of the forces involved is not the only feature that has turned heads.

Forces from China and Mongolia also plan to take part. Beijing has said it will send about 3,200 troops, 30 helicopters, and more than 900 other pieces of military hardware.

China's Defense Ministry said the drills were meant to strengthen the two countries' strategic military partnership and increase their ability to respond to threats and ensure stability in the region.

The Kremlin spokesman Dmitry Peskov said China's participation "speaks about the expansion of interaction of the two allies in all the spheres."

Chinese forces have already joined their Russian counterparts in some military exercises.

This month, Chinese forces are taking part in Peace Mission 2018, an exercise organized by the Shanghai Cooperation Organization, a regional bloc led by Russia and China. (It's the first exercise to include all eight SCO members.)

But including China in the Vostok exercise hints at a significant geopolitical shift.

"China was seen as the potential threat or target in exercises like Vostok," Alexander Gabuev, an expert on China at the Carnegie Moscow Center, told The New York Times.

"But it is now being invited to join as a friend and even a quasi-ally," Gabuev added. "This is really unprecedented."

The Soviet Union clashed with China along their shared border several times in the 1960s — once in a deadly Chinese raid on a Soviet border outpost that almost kicked off a full-scale war in early 1969.