What was Avimelech worried about that prompted him to enter into a covenant with Yitzchak?
Ramban #1, Moshav Zekenim: He was worried (not that Yitzchak would attack him, but) that, because he (Avimelech had abrogated the covenant that they had made with Avraham (by expelling him from Grar), Yitzchak's descendents would do likewise and drive his children out of their land. So he entered into a new covenant with Yitzchak. 1
Ramban #2: Avimelech entered into a covenant with Avraham, because he was afraid of Avraham, who had proven himself to be a mighty warrior on whose side Hashem fought, and who a household consisting of three hundred swordsmen, might attack him. Moreover, he may well have heard about Hashem's promise to give Eretz Kena'an to Avraham's offspring. And it is for the same reason that he entered into a new covenant with Yitzchak - who, like his father, was as great as his father, and of whom he likewise feared.
Playing down the fact that he had abrogated the original one with smooth talk ('We did only good with you!').
What did the Shevu'ah that they made to each other incorporate?