On 8 February 1963, a military coup in Iraq led by a Ba'athist–Nasserist alliance toppled Qasim, who was subsequently shot dead. Abdel Salam Aref, a Nasserist, was chosen to be the new president. A similar alliance toppled the Syrian government on 8 March. On 14 March, the new Iraqi and Syrian governments sent Nasser delegations to push for a new Arab union. At the meeting, Nasser lambasted the Ba'athists for "facilitating" Syria's split from the UAR, and asserted that he was the "leader of the Arabs". A transitional unity agreement stipulating a federal system was signed by the parties on 17 April and the new union was set to be established in May 1965. However, the agreement fell apart weeks later when <answer>Syria's Ba'athists purged Nasser's supporters</answer> from the officers corps. A failed counter-coup by a Nasserist colonel followed, after which Nasser condemned the Ba'athists as "fascists".
What caused the agreement to fail?
Syria's Ba'athists purged Nasser's supporters