The "covenant" referred to in Daniel 9:27 is evidently the same one referred to in 1 Maccabees:
"From them came forth a sinful root, Antiochus Epiphanes, son of Antiochus the king; he had been a hostage in Rome. He began to reign in the one hundred and thirty-seventh year of the kingdom of the Greeks. In those days lawless men came forth from Israel, and misled many, saying, "Let us go and make a covenant with the Gentiles round about us, for since we separated from them many evils have come upon us." This proposal pleased them, and some of the people eagerly went to the king. He authorized them to observe the ordinances of the Gentiles" (1 Maccabees 1:10-13).
This covenant, or "alliance", was the arrangement that precipitated the whole persecution and sacrileges during the reign of Antiochus. As a result, the "abomination of desolation" was established in the Temple as described in 1 Maccabees 1:54. As Josephus says, "the king built a pagan altar upon the temple-altar and slaughtered swine thereon, thereby practicing a form of sacrifice neither lawful nor native to the religion of the Jews" (Antiquities, 12.5.4). Similarly, with reference to Antiochus' rededication of the Temple to Zeus Olympius (Baal-Shamem), 2 Maccabees 6:5 says that "the altar was covered with abominable offerings that were forbidden by the Law". That this is what the phrase "abomination of desolation" referred to is indicated by the fact that Hebrew shqwts shmm "abomination of desolation" is a direct pun on "Baal-Shamem", the name of the deity that Antiochus had placed in the Temple. Here shqwts "appalling, abomination" is used as a substitution for Baal (cf. bsht "shame" as a substitution in Hosea 9:10, Jeremiah 3:24, 11:13), and shmm "causing horror, desolation" is similarly a pun on shmym "Shamem (= heaven)".
Because Antiochus had installed the "abomination of desolation," he is referred to as "the desolator" (shmm) in Daniel 9:27, tho the LXX has "desolation" instead (cf. the abomination that had been installed) and the Vulgate has "desolation that will endure until and consummation and the end". The Hermeneia translation by Collins reads: "The host of a ruler who is to come will destroy the city and the sanctuary. His end will be in a cataclysm and unto the end of the decreed war there will be desolations. He will make a strong alliance with the multitude for one week (= 7 years). For half the week (= 3 1/2 years) he will suppress sacrifice and offering, and the desolating abomination will be in their place, until the predetermined destruction is poured out on the desolator" (Daniel 9:26-27).