China and the Soviet Union had been the principal backers of North Vietnam's effort through large-scale military and financial aid. The two communist superpowers had competed with one another to prove their "fraternal socialist links" with the regime in Hanoi. The North Vietnamese had become adept at playing the two nations off against one another. Even with Nixon's rapprochement, their support of North Vietnam increased significantly in the years leading up to the U.S. departure in 1973, enabling the North Vietnamese to mount full-scale conventional offensives against the South, complete with tanks, heavy artillery, and the most modern surface-to-air missiles.