In July of 1965, in conjunction with the Marine Corps establishing a presence in South Vietnam, 4th Battalion 12th Marines came ashore setting up headquarters at Phu Bai, establishing it's TAOR.
At Phu Bai, after establishing it's command location near the airstrip, 4/12 took operational and administrative control of the artillery presence in place. These were one of its own 155mm howitzer batteries, Battery M; a 105mm battery, Battery I, 3d Battalion 12th Marines and Whiskey/Mortar Battery, a 4.2 unit from 2d Battalion 12th Marines.
On 16 September, Battery M received six of the newer M-109 155mm self-propelled howitzers; its older M-114A towed pieces were then distributed throughout the artillery battalion. Headquarters Battery and Batteries I and M each manned two of the towed 155s. Lieutenant Colonel Sumner A. Vale later remarked:
. . . "seldom if ever has an infantry battalion commander had so much artillery support under his control as did Taylor, I, and then Hanifin .... We had the 105 battery within the BLT organization, the equivalent of] two batteries of 155 howitzers, one towed and one self-propelled, and a battery of howtars .... These 24 artillery pieces compensated, in part, that 3/4 had only 3 rifle companies, one being stationed in the Da Nang area. "
A Marine division had a variety of available artillery support. Its artillery regiment consisted of three direct support and one general support battalions. The three direct support battalions, 1st, 2d, and 3d, contained three batteries each with six M101A1 105mm towed howitzers (range 11,300 meters), and one battery of six 107mm howtars (range 5,600 meters), a 4.2-inch mortar tube mounted on the frame of the old 75mm pack howitzer.
4th Battalion, the general support unit, had three batteries each equipped with six 155mm howitzers (range 14,600 meters). In 1965 M-109 self-propelled 155mm howitzers were being phased in to replace the older M114A towed howitzers. 4th Battalion, 12th Marines had deployed to Vietnam with two batteries equipped with self-propelled howitzers and one battery with towed howitzers.
In Vietnam, the Marines found they had a use for both weapons. The heavy, tracked M109SP was largely road bound, while the lighter towed howitzer could be moved either by truck or by helicopter allowing more mobility for support in the field. War for the 12th Marines was underway...