Complex sentence: It was not until 1988 that South Africa agreed to implement the resolution, signing the Tripartite Accord (an agreement between Angola, Cuba and South Africa) at UN headquarters in New York.
Split into simpler sentences: It was not until 1988 that South Africa agreed to implement the resolution. This occurred when South Africa signed the Tripartite Accord (an agreement between Angola, Cuba and South Africa) at UN headquarters in New York.