The battalion distinguished itself in the defense of Khe Sanh during the Vietnam War, and later participated in an ill-fated invasion of Koh Tang Island in Southeast Asia, with the intention of rescuing the crew of SS Mayaguez.

The battalion helped evacuate Americans from Northern China during the Chinese Civil War and in various occasions participated in providing relief to the victims of typhoons.

Each company had its own weapons platoon with 3 squads of M60 machine guns (7.62) and 60 mm mortars and ether 3.5-inch rocket launchers (super bazooka) or later LAAWS.

The battalion's main objective was to train reserve Marines and its headquarters was now transferred to Kansas City, Missouri, with two companies stationed at St. Louis.

[3] In April 1942, five months after the Japanese attack on Pearl Harbor, the unit was activated as part of the 2nd Marine Division.

[3][4] The 2nd Battalion, 9th Marines were part of the reserve forces for the Battle of Iwo Jima and were committed to the action five days after D-Day.

[3] On June 25, 1950, war broke out between the provisional governments of North and South Korea as they competed for control over the Korean peninsula.

2nd Battalion, 9th Marines fought battles in or around Danang, Hue, Phu Bai, Đông Hà, Camp Carroll, Cam Lộ, Con Thien, Than Cam Son, Quảng Trị, Cửa Việt, Vandegrift Combat Base and what is considered by many as their most vicious engagement, the Battle of Khe Sanh.

It was used as a staging ground for a number of attacks on People's Army of Vietnam (PAVN) troop movements down the Ho Chi Minh Trail.

[6] From January 22 to March 18, 1969, 2/9 participated in Operation Dewey Canyon which was a sweep of the A Shau Valley and the last major offensive by the Marine Corps in Vietnam.

[7] On August 1, 1969, 2/9 departed Vietnam onboard the USS Paul Revere bound for Camp Schwab, Okinawa.

[8] During this period the unit was assigned to sea duty in and around the waters of Vietnam and continued to receive combat training at Camp Fuji, Japan and Subic Bay in the Philippines.

[3] On May 12, 1975, barely two weeks after the fall of Saigon, Khmer Rouge forces seized a U.S. flagged merchant ship, the SS Mayaguez in recognized international sea lanes claimed as territorial waters by Cambodia and removed its crew for questioning.

The Khmer Rouge naval forces used abandoned US Navy "Swift Boats" in the seizure of the U.S. container ship.

[9] Calling the seizure "piracy", President Ford ordered a military response to retake the ship and its 39-man crew, mistakenly thought to be on Koh Tang Island.

The Marines and the CH-53 helicopters which transported them, were attacked by the Khmer Rouge with machine guns, mortars, and rocket propelled grenade launchers in what became known as the Battle of Koh Tang Island.

[9] The Mayagüez incident with the Khmer Rouge, which ended on May 15, 1975, marked the last official battle of the 2nd Battalion, 9th Marines in the Vietnam War.

In October 1990 the unit deployed to the Republic of the Philippines where it became the Ground Combat Element of Marine Air-Ground Task Force (MAGTF) 4-90.

Dictator Mohammed Siad Barre had deliberately broken up the armed forces into clan factions in order to maintain his hold on power, starting from 1987.

In August 1992, President George H. Bush sent, 25,000 US troops (mostly US Marines from I MEF) to the Republic of Somalia and the mission was renamed Operation Restore Hope, also known as UNOSOM II.

BLT 2/9's mission was to be the lead unit, Marine Raiders from Fox Company performing the first live rubber boat landing since WW2 secured the seaport and a force Recon unit followed by Gulf company secured the airfield in Mogadishu which allowed the rapid build-up of forces in-country.

On January 30, 1993, a Marine patrol was ambushed in Mogadishu by gunmen faithful to warlord Mohammed Farah Aidid bringing about casualties.

[15] In September 2008, Marines and Sailors from 2/9 deployed to Al Anbar Province in support of Operation Iraqi Freedom.

July 2010 Marines and Sailors from 2/9 deployed to Marjah, Helmand Province, Afghanistan in support of Operation Enduring Freedom.

[16] In December 2010 Echo Company from 2/9 were attached to 3/5 in Sangin, Afghanistan where they earned the notorious nickname of "Green Hats."

Echo and Weapons companies deployed once more to Afghanistan from January through April 2013, participating in combat operations out of Camp Leatherneck.