Original link: Trump, in show of NATO support, nominates official to key role with alliance / Politico.
The tradition is that the political lader of NATO is a European while the military leader is an american.
Next SACEUR (Supreme Allied Commander EURope) will be Alexus Grynkewich USAF replacing Chris Cavoli USA.
It is a rather prestigious post within the US military and former holders include one president and one US secretary of state (Eisenhower and Haig). It is not a post with a high public profile though and many may never have heard about it.
But if Russia attack a NATO member, then SACEUR will take operational command of all NATO forces in Europe and be responsible for defense and counter attacks. He will be leading an army of approx. 2 million soldiers into battle.