Twenty thousand American troops are preparing for their biggest transatlantic military deployment in 25 years.
Exercise Defender Europe 20 is set to last several months and will involve more than 35,000 personnel from across 18 nations, including the UK.
Britain will be the largest non-US contributor to the exercise with 2,500 soldiers.
Major General Andrew Rohling, Deputy Commanding General of the US Army Europe, said: "This is an exercise in defence, this is an exercise in crisis preparation and an exercise in strategic readiness."
The exercise follows Moscow's annexation of Crimea and accusations of involvement in the Ukraine conflict.
But Major General Rohling said: "It has nothing to do with any specific country."
The US aims to use the exercise to project its power and test its ability to rapidly deploy a large force to support NATO in response to a crisis, as well as to send a signal to any potential adversaries.
Parts of continental Europe will provide the setting for a simulated battlefield and ten countries will be involved in a combat scenario.
The US 82nd Airborne Division will launch assaults by parachuting into Lithuania, Georgia and Poland.
The British-led Allied Rapid Reaction Corps will co-ordinate land operation, and the UK will provide logistic, medical, and helicopter support.
While some units will bring their own vehicles and equipment, others will collect tanks and artillery from locations in Belgium and Germany.
Also active in Defender-Europe 20 will be the Royal Engineers, including the only remaining sappers based in Germany.
By April, 9,000 American soldiers already stationed in Europe will have joined the exercise.
A further 8,000 from 17 other NATO countries will take the total to 37,000 before 20,000 US soldiers return home in June.
Joint Helicopter Command Apaches will also be at the forefront of the British effort, as well as 700 personnel from 104 Logistic Brigade and 450 British Army medics will also provide support.
America is also preparing a similar exercise later this year in the Pacific.