VUNG TAU, Vietnam — The 25th Inf. Div. became the largest U.S. combat force in Vietnam when the 196th Light Inf. Brigade arrived here Sunday to join up with the Tropic Lightning Div.

With the addition of the 4,000 troops of the brigade, the 25th becomes the. largest U.S. combat force in: the Republic. U.S. troops number some 292,000.

The 196th will set up its headquarters at Tay Ninh. about 40 miles northwest of Cu Chi. Cu Chi is the home of the 25th Inf. Div.'s 1st and 2d Brigades. The 3d Brigade is located at Pleiku. Tay Ninh is about 70 miles northwest of Saigon.

The brigade was led ashore by Capt. Cecil Vaughn, B Co., 3d Bn., 21st Inf.

Lt. Gen. Jean E. Engler, deputy commanding general, U.S. Army Vietnam, welcomed the troops.

Brig. Gen. Edward Desaussure, former deputy commander of the 25th Inf. Div., is commander of the brigade.

The brigade is the first "light" infantry brigade in American military history. Basically, the designation means that the brigade as a whole is capable of operating with less gear than other infantry divisions.

For example, most infantry units carry artillery as big as 155mm, but the new brigade will only employ 105mm, thus giving them greater mobility.

The brigade was activated Sept, 15, 1965, at Fort Devens, Mass. The unit left the U.S. after receiving eight months of training.

Col. Francis S. Conaty Jr., deputy commander of the brigade said that about 50 per cent of his troops, or 2,000 men, were draftees.

The brigade will be given a brief period of in-country training and then will be turned out in "spoiling operations," which are now keeping most U.S. strike battalions almost continually in the field.