Field marshal is the most senior military rank, ordinarily senior to the general officer ranks. Usually it is the highest rank in an army and as such few persons are appointed to it. It is considered as a five-star rank in modern-day armed forces in many countries.

What powers does a field marshal have?

They serve only for a single battle, the position of Field Marshal has no role in the ongoing military campaigns involving the Imperial armies. The position has few legal powers but it is prestigious - nearly half of those who have sat on The Throne have served as Field Marshal at some point during their life.

Why doesn't the US Army have field marshals?

The reason was ultimately that the name of U.S. Army Chief of Staff General George C. Marshall would have sounded ridiculous with this new rank. Still, the Americans were now the senior partner in the alliance against the Axis and its commanders were technically outranked by British Field Marshals.