Field Marshal is the highest military rank of many countries, but not of the United States. To become a field marshal, one must capture a significant city (Field Marshal Erwin Rommel of Tobruk or Field Marshal Erich von Manstein of Sevastopol), a significant area (Field Marshal Wavell of Cyrenaica), or win a significant battle (Field Marshal Bernard Montgomery of El Alamein).
Just imagine if the United States Army had had such a rank during the Civil War. We might have had Field Marshal Grant of Fort Donelson and Vicksburg, Field Marshal Burnside of Pamlico Sound, Field Marshal Butler of New Orleans, Field Marshal Buell of Nashville, Field Marshal Banks of Port Hudson, Field Marshal Rosecrans of Chattanooga, Field Marshal Sherman of Atlanta and Savannah, Field Marshal Terry of Fort Fisher, and Field Marshal Canby of Mobile. President Lincoln would probably have had to promote Little Mac to Field Marshal of Antietam to justify issuance of the Emancipation Proclamation.
Just imagine if the United States Army had had such a rank during the Civil War. We might have had Field Marshal Grant of Fort Donelson and Vicksburg, Field Marshal Burnside of Pamlico Sound, Field Marshal Butler of New Orleans, Field Marshal Buell of Nashville, Field Marshal Banks of Port Hudson, Field Marshal Rosecrans of Chattanooga, Field Marshal Sherman of Atlanta and Savannah, Field Marshal Terry of Fort Fisher, and Field Marshal Canby of Mobile. President Lincoln would probably have had to promote Little Mac to Field Marshal of Antietam to justify issuance of the Emancipation Proclamation.