WASHINGTON, January 29 -- The United States suddenly ended today a year's effort to mediate between China's warring factions and announced that most of the remaining Marines and Army forces would be pulled out shortly.

Secretary of State Marshall's action was announced simultaneously here and at Nanking.

It put squarely up to the Chinese themselves the task of working out their salvation and raised a big question mark over future United States policy toward China.

Both among diplomats here and at Nanking it was viewed as a likely signal for the start of the full scale civil war long brewing between Chiang Kai-Shek's Nationalist government and the Communists in North China.

Nevertheless some hope remained that the move would spur the nationalists into giving their government a broad base of "liberal" elements as urged by Marshall upon his return this month to become Secretary of State.

"The assumption of leadership by the liberals in the government," Marshall said then, would be "the salvation of the situation."