Complex sentence: By July 1974, the Vietnamese leadership had decided to set the the victory year to 1975, instead of 1976 as originally planned, because they believed an earlier Vietnamese unification would put Vietnam in a stronger position against Chinese and Soviet influence in the region.
Split into simpler sentences: By July 1974, the Vietnamese leadership had decided to set the the victory year to 1975, instead of 1976 as originally planned. They did this because they believed by quickening up Vietnamese unification, believing that it would put Vietnam in a stronger position against Chinese and Soviet influence in the region.