cavalry

A component of many armies, cavalry refers to units of soldiers mounted on horseback. Historically cavalry units were very important in military operations due to the advantages of mobility and speed afforded by horses. Beginning in the late 1800s and continuing into the next century, the role for cavalry in the U.S. diminished with the introduction of new weapons. By the 1950s horses were no longer used by the U.S. Army, although some units are maintained ceremonially as part of state militias. The 1st Cavalry Division nevertheless served extensively in Vietnam using helicopters to transport weapons and vehicles.

This term is often used colloquially to mean the arrival of extra help to rescue a person or situation, as in "the cavalry has arrived!"