There will be a union between the two empires that emerged from the Greek Empire. A covenant is made between Ptolemy of Egypt and Antiochus II who rules from the north. The confirmation of that covenant is the marriage between Antiochus and the Egyptian princess Bernice. This marriage is intended to ensure the peace of which we read here. Yet that peace will not last. The second part of this verse is fulfilled as well.
The Egyptian king dies. Then Antiochus reconciles with his first wife, Laodice.
Yet this first wife has not forgotten her previous humiliation. She poisons Antiochus. She also has the child of Antiochus and Bernice put to death. Then Bernice and her entire Egyptian court are murdered. Everyone involved in the peace between the two empires is killed.
The relatives in Egypt are not just going to let this happen.
6 After some years they shall make an alliance, and the daughter of the king of the south shall come to the king of the north to make an agreement. But she shall not retain the strength of her arm, and he and his arm shall not endure, but she shall be given up, and her attendants, he who fathered her, and he who supported her in those times.