The Supreme Allied Commander position in NATO allows the US to lead allied forces in wartime. However, the Trump administration is considering withdrawing the US from that position (mainly for cost-cutting reasons).
Stepping away from the Supreme Allied Commander position would signal America’s withdrawal from NATO, since US forces cannot legally be placed under foreign command. There must be some strategic misunderstanding of the power this title holds, a lack of expertise in Trump’s circle, a penetration into Trump’s thinking by Russian propaganda, or a combination of all of those.
Should the US move forward with leaving NATO leadership, US power projection in Europe would be crippled and another box on the Russian wish list would be crossed off.