Alarmed by the new militancy of the Chinese and perhaps also responding to Mao’s 1964 challenge to Moscow on the territorial issue, the Soviet Union began to upgrade its armed forces along the lengthy border with China. Perhaps mindful of the claims on Mongolia that Mao had raised with Khrushchev in 1954, despite having recognized its independence four years earlier, the Soviet Union signed a defence treaty with Mongolia in 1966 and by the following year had stationed several divisions there. The growing tension and pressure from the militarily superior Soviet forces led the Chinese to attack one of the disputed islands in the Ussuri River on the Manchurian border in March 1969. The Soviet response, which involved penetrations across the border into Xinjiang, eventually compelled the Chinese to negotiate in September.