The correct answer is either manifest destiny or the civil war
Because of the civil war and the expenses of maintaining a navy that could blockade Confederate ports, the Navy was constantly losing power and ships and after the war ended most of the 700 initial ships were completely out of service. As manifest destiny and reconstruction began spreading, the government did not focus on rebuilding its navy and a large part of the navy that remained had to focus on things like helping merchants or doing civilian runs in order to be sustained.
Indochina, Only Indochina really repelled the Mongols.
