U.S. military personnel will focus on how to respond to aggressions from China and Russia when they play a highly classified war game this summer.

The game is a key priority for Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, who is leading the exercise, with Defense Secretary Lloyd Austin to be briefed while it plays out, according to several defense officials who spoke with CNN. 

The war game will focus on a fictional global crisis evolving on multiple fronts and involve players competing for military assets and dealing with unexpected moves and changing scenarios. 

The results of such games are closely guarded, as they can reveal shortfalls, but one former defense official confirmed that in an exercise concerning a fictional conflict against adversaries like Russia and China, "we found the Blue Team, the US and allies, kept losing."

The exercise is taking place when the military's budget is being set and decisions on troop levels and other priorities are being considered and comes just months after President Joe Biden's inauguration. 

Some of the war game's scenarios are expected to reflect real-life potential events, including cyber attacks, Russia advancing in the Baltics, a Chinese invasion of Taiwan, or Chinese actions in the South China Sea or an invasion of another country, like Taiwan. 

Meanwhile, the preparation for aggression is not only virtual. This week, the United States and Canada have been conducting military exercises in conditions where the temperatures can get to 20 degrees below zero Fahrenheit to demonstrate they can push back against advances by the Russian military in the Arctic. 

Russia has already installed advanced missiles in the region and has been challenging the United States, flying more aircraft in 2020 near the U.S. airspace off Alaska than since the end of the Cold War, according to the North American Aerospace Defense Command. The flights have included intelligence collection planes, heavy bombers, and anti-submarine aircraft. 

Canadian NORAD Region Commander, Major-General Eric Kenny, told CNN that for NORAD, a key priority for the exercise is "being able to track and then defeat" potential Russian military activity in the Arctic. 

In addition, Eric Edelman, a former defense undersecretary for policy and expert on military planning, told CNN that "Russia and China military modernization creates some serious potential operational challenges for the US," as both countries are expanding abilities to operate in wider areas of Europe and Asia. 

This could mean the Pentagon could be forced to send U.S. forces further from home, and that "Russia and China are playing a home game, (but) we are playing an away game," said Edelman.

The war games are being planned as rhetoric from the Biden administration heats up concerning both China and Russia. 

This week, Secretary of State Antony Blinken called out Russia during a NATO meeting in Brussels for "reckless and adversarial actions," saying Moscow has "built up forces, large scale exercises and acts of intimidation, in the Baltic and the Black Sea."

Deputy Defense Secretary Kathleen Hicks, meanwhile, said Beijing has "demonstrated increased military competence and a willingness to take risks" and that its actions "constitute a threat to regional peace and stability, and to the rules-based international order on which our security and prosperity and those of our allies depend."

The games also come while both countries are upgrading their nuclear capabilities, top commanders are warning. 

Admiral Charles Richard, head of the U.S. Strategic Command, wrote in a recent article in the Proceedings of the US Naval Institute journal that Russia is upgrading its "entire strategic force structure," including bombers, intercontinental ballistic missiles, and warning systems. 

He also warned that China is about to become a full nuclear triad that has nuclear-capable missiles, submarines and will soon have a long-range bomber. 

China has had a "No First Use" policy since the 1960s, he wrote, but that could "change in the blink of an eye," said Richard. 

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