Complex sentence: Knowing the proposal was a threat to their countries borders, the Greek and Yugoslav delegates were unable to endorse it at this stage; however, in order for any chance of success, the communists needed the support of the IMRO.
Split into simpler sentences: Knowing the proposal was a threat to their countries borders, the Greek and Yugoslav delegates were unable to endorse it at this stage. However in order for any chance of success, the communists needed the support of the IMRO.