In ancient times among some peoples such as so call barbarian tribes and nations women fought alongside the men.
Some rose to great fame and commanded nations, armies and navies like Boudica, Joan of Arc, Artemisia, Grace O'Malley and others.
Joan Of Arc
World War 1
It was during the first world war that women were mobilized in unprecedented numbers mostly to replace the men in industrial and agricultural work who were conscripted to fight. Tens of Thousands of women served as nurses in the military behind the lines in every theatre.
Russia was the only country to use female soldiers. Its few "Women's Battalions" fought well, In the later Russian Civil War, the Bolsheviks would also employ women infantry.
Individual women would fight in various revolutions and conflicts between ww1&ww2.
World War 2
More women were mobilised than ever during WW2 espicially in factories and agricultural. Women played ever more important roles in the military freeing up more men for combat duties these roles included administration, nurses, truck drivers, mechanics, electricians, and auxiliary pilots but also 700,000 served in combat roles mostly as Anti-aircraft gunners. In Britain women had important roles such as aircraft controllers which was so important during the Battle of Britain, as spies and radio operators for SOE parachuting into nazi occupied Europe, or as decoders, decripting the German enigma code and interpreting aerial photographs.
Even future Queen Elizabeth served as a driver/mechanic in the British Army.
In all, 350,000 American women served in the U.S. military during World War II. Women didn't have some of the roles in the military that women had in Britain but they greatly helped the war effort. But language speicialists did help intelligence agencies greatly. Women also were greatly involved as decoders with MAGIC and were parachuted into occupied Europe as OSS spies like their British counterparts. A large number of American women served as Nurses all over the world in the US military during WW2.
More than 800,000 women served in the Soviet Armed Forces during the war, which is roughly 3 percent of total military personnel, mostly as medics. About 300,000 served in anti-aircraft units and performed all functions in the batteries—including firing the guns. Some Russian women also served as bomber pilots the most famous of these were called the Night Witches.
Famously the Soviet Union also deployed women as snipers on the brutal eastern front. One of these was Lyudmila Mikhailovna Pavlichenko credited with 309 kills. She is regarded as one of the top military snipers of all time and the most successful female sniper in history.
Other countries employed women in various roles which I won't go into but Britain, America and the Soviet union were the major users of women in the military.
Israel has been a big user of women in the military since its birth. Current women are conscripted into the military just like the men and combat roles have long been open to women.
In Vietnam thousands of women fought french and American troops in the Vietcong.
Cold war and post cold war US military.
During the cold war women gained more roles in the military for instance In 1974, the first six women aviators earned their wings as Navy pilots, On December 20, 1989, Captain Linda L. Bray, 29, became the first woman to command American soldiers in battle, during the invasion of Panama. Over 40,000 women served in almost every role the armed forces had to offer in the 1991 Gulf War which was a pivotal time for women in the US armed forces.
Afganistan and Iraq.
Women's roles in the military have changed dramatically since 2001 with more roles open to women.
During the Iraq War and the Afghanistan War, more than two hundred thousand women served, of which 152 were killed; of those 84 were killed by enemy action. During the Afghanistan War, American soldier Monica Lin Brown, was presented the Silver Star for shielding wounded soldiers with her body, and then treating life-threatening injuries.
In 2008, Ann Dunwoody became a four-stargeneral in the Army, making her the first woman in U.S. military and uniformed service history to achieve a four-star officer rank. In 2011, Major General Margaret H. Woodward commanded Operation Odyssey Dawn's air component in Libya, making her the first woman to command a U.S. combat air campaign. In 2012, Janet C. Wolfenbarger became the first female four-star general in the Air Force.
Michelle J. Howard began her assignment as the U.S. Navy's first female (and first African-American female) four-star admiral on July 1, 2014. In December 2015, Defense Secretary Ash Carter stated that starting in 2016 all combat jobs would open to women. In March 2016, Ash Carter approved final plans from military service branches and the U.S. Special Operations Command to open all combat jobs to women, and authorized the military to begin integrating female combat soldiers "right away."
Women have been in combat since 2001 in both Iraq and afganistan not as frontline infantry soldiers but as drivers in convoys and other non infantry roles but combat is not just on the frontlines. Now that combat roles in the infantry are open to women the real question is can they be as effective as men. A big hinderance is the weight of the gear an infantry soldier must carry which includes a rifle, body armor, ammo and pack etc which weighs between between 60 and 100 pounds and the average woman at 5.3ft weighs 168.5 pounds. Male infantry soldiers are currently having trouble with injuries to their backs, feet, ankles and knees from humping that gear for miles while training and on deployment so it must be worse for women as we are biologically weaker physically. I'm all for women in the infantry provided the bar is not lowered for us due to our gender.