Merrill's Marauders, a Ranger type outfit, came into existence as a result of the Quebec Conference of August, 1943. During this conference, President Franklin D. Roosevelt, Prime Minister Winston Churchill of England, and other allied leaders conceived the idea of having an American ground unit spearhead the Chinese Army with a Long Range Penetration Mission behind enemy lines in Burma. Its goal would be the destruction of Japanese communications and supply lines and generally to play havoc with enemy forces while an attempt was made to reopen the Burma Road.

A Presidential call for volunteers for "A Dangerous and Hazardous Mission" was issued, and approximately 2,900 American soldiers responded to the call. Officially designated as the 5307th Composite Unit (Provisional) code name "GALAHAD" the unit later became popularly known as MERRILL'S MARAUDERS, named after its leader, Brigadier General Frank Merrill. Organized into combat teams, two to each battalion, the Marauder volunteers came from a variety of theatres of operation. Some came from Stateside cadres; some from the jungles of Panama and Trinidad; and the remainder were battle-scarred veterans of Guadalcanal, New Georgia, and New Guinea campaigns. In India some Signal Corps and Air Corps personnel were added, as well as pack troops with mules.

After preliminary training operations undertaken in great secrecy in the jungles of India, about 600 men were detached as a rear echelon HQ to remain in India to handle the soon-to-be vital air-drop link between the six Marauder combat teams (400 to a team) and the Air Transport Command. Color-coded Red, White, Blue, Green, Orange and Khaki, the remaining 2400 Marauders began their March up the Ledo Road and over the outlying ranges of the Himalayan Mountains into Burma. The Marauders, with no tanks or heavy artillery to support them, walked over 1,000 miles throughout extremely dense and almost impenetrable jungles and came out with glory. In five major and thirty minor engagements, they defeated the veteran soldiers of the Japanese 18th Division (conquerors of Singapore and Malaya) who vastly outnumbered them. Always moving to the rear of the main forces of the Japanese, they completely disrupted enemy supply and communication lines, and climaxed their behind-the-lines operations with the capture of Myitkina Airfield, the only all-weather airfield in Burma.

For their accomplishments in Burma, the Marauders were awarded the Distinguished Unit Citation in July, 1944. However, in November, 1966, this was redesignated as the PRESIDENTIAL UNIT CITATION which is awarded by the President in the name of Congress. The unit was consolidated with the 475th Infantry on August 10, 1944. On June 21, 1954, the 475th was redesignated the 75th Infantry. It is from the redesignation of Merrill's Marauders into the 75th Infantry Regiment that the modern-day 75th Ranger Regiment traces its current unit designation.

In 1969 when the Vietnam era Ranger companies became the 75th Rangers the lineage of the WWII Ranger Battalions was held by the Army Special Forces.  That lineage has scince been returned to the 75th Ranger Regiment making them historical decendents of Merrills Marauders, the WWII Ranger Battalions, and the Korean War Ranger Companies.

My understanding is that the 75th Ranger Regiment in it's entirity now carries the lineage of:

 

- The 75th Infantry Regiment; previously the 475th Infantry Regimant and also the 5307th Composite Unit (Provisional) aka Merrill's Marauders before that.

- The six WWII Ranger Battalions.

- The (number?) Korean War Airborne Ranger Companies.

 

I don't know if the three battalions of the 75th somehow have different pieces of the lineage separated out among them.

I believe that at the start of the Korean war , there were 4 ranger companies formed from the ranks of the 505th Parachute regiment as the Rangers were actually disbanded post WWII, during the Korean conflict there was one ranger company of 112  per 18,000 man division.

 

13 ranger comapnies existed going into Vietnam which were aligned with the 75th and as Longrifle said in 1969 became the 75th Ranger Reg. I have a source for it somewhere Ilet me find it