4 incredible women in military history you need to know about

International Women's Day has been celebrated across the world since 1909, and is used as a day to laud the important contributions women make.

Women have long-since served in the U.S. military, even before they were officially allowed to enlist. From covert spy operations to battles on the front lines, women have been there for all of it.

Nancy Morgan Hart

During the American Revolution, Hart was supposed to stay and take care of her children at their Georgia home while her husband fought in the war, like many military spouses today do. However, Nancy couldn't sit idly by while a war raged around her.

Related: The only 7 women to receive the Distinguished Flying Cross

Pretending to be a crazy man, Hart was able to gain access to British camps in Augusta, where she successfully gathered intelligence and reported it back to the Continental Army. Hart also wasn't afraid to defend her home against the enemy, as evidenced when six Loyalist soldiers entered her home and demand she feed them. While they were occupied with food, she hid their weapons and held them hostage with one, killing two when they tried to overpower her, until her husband and a neighbor came home.

Dr. Mary E. Walker

Walker volunteered her expertise as a surgeon with the Union Army at the beginning of the Civil War, despite women not being allowed to serve as doctors. She was captured and became a prisoner-of-war after she was caught crossing enemy lines to treat wounded soldiers. She was considered a spy by the Confederates and was held until eventually released in a prisoner exchange.

For her bravery and willingness to confront the enemy to save Union soldiers, President Andrew Johnson awarded her the Medal of Honor, after a recommendation by Gen. William Sherman, becoming the first and only women ever to be awarded the highest military honor.

Col. Eileen Collins

Collins became the first female to pilot a shuttle in space in 1995, and was also the first female commander of a U.S. spacecraft in 1999.

During her time in the Air Force, Collins served as an instructor for the T-38 Talon at Vance Air Force Base, and eventually transitioned to an assistant professor role at the U.S. Air Force Academy, teaching mathematics and instructing T-41 pilots.

Sarah Emma Edmonds

Edmonds fled to Michigan from Canada, escaping an abusive marriage. While traveling, she found that dressing like a man made life considerably easier, and eventually joined the military as a male nurse out of a sense of obligation. Edmonds used the alias "Franklin Thompson," and served as a spy for Union soldiers until she was confronted with a bout of Malaria. Knowing she would be punished if Army doctors discovered she was a woman, Edmonds abandoned her male disguise and continued to serve as a female nurse in Washington D.C.

After she wrote a memoir about her time as a spy, Edmonds contributions to the war were accepted, and she received an honorable discharge, as well as a government pension for her service.