The Seleucid Empire, the ruling culture being essentially Greek (it was a portion divided from the conquests of Alexander the Great), was based in Antioch. It held nominal control over the Levant, northern Arabia, Mesopotamia, and Persia, and it aspired to control Egypt, Greece, and Macedonia. It was hemmed in by smaller rivals, including Ptolemaic Egypt and a newer, more vital power: Rome. It was also hampered by rebellious tributary states and subjects, such as the Bactrians, the Parthians, and the Jews (among many others). After Macedonia was sidelined by Rome in 197, the Seleucid Emperor saw his chance to invade and conquer Greece. The Romans reacted defensively and defeated him in 190, imposing a Versailles-like treaty that required the Seleucid Empire to limit its military strength and pay a heavy indemnity. The Emperor died attempting to collect funds from vassals in the east so he could pay the Roman indemnity in 189, and his son continued that task until he was assassinated by his own minister in 175.
Then things began to spin out of control when succession was claimed by a brother, rather than a son. (The son was being held hostage by the Romans.) The brother attacked Egypt and was just about to seize it when a Roman commissioner appeared at his tent and forced him to give up and retire to Antioch, simply by threatening him with the Senate's wrath. Frustrated, the brother spent the rest of his reign concentrating on controlling his restive subjects, including forcing Hellenistic religious reforms onto the Jews, which resulted in the Maccabean revolt starting in 167. His loss of face following the Roman intervention in Egypt also resulted in Parthian seizure of a portion of his Persian lands, and he died fighting them there in 164. Following his death Seleucid army operations against the Maccabees ceased while affairs in Antioch resolved themselves.
Upon the brother's death, the Romans contrived with parts of the Seleucid court to place his nine year-old son on the throne, with a trusted Seleucid general as his regent. These two sent another army to deal with the Maccabees, but it met with mixed results. Judas Maccabeus managed to defeat the regent in 164 and drive the Seleucids and those Jews who favored Hellenistic practices out of Jerusalem. This setback was compounded by the arrival in Antioch of a rival regent who had accompanied the erstwhile Emperor (the brother) to Persia. The rival regent took control of Antioch, forcing the boy-Emperor and his regent to offer a peace treaty to the Maccabees, which they accepted. Untrusting, the Emperor had the walls of Jerusalem demolished before he withdrew to settle affairs in Antioch. Occurring simultaneously was another conflict with Rome: When they learned the Empire was not complying with the specified limits on ships and war elephants, the Romans sent a heavily-armed envoy who sank ships and hamstrung elephants until the Seleucids finally had him assassinated in 162. By 162, dissatisfaction with the Seleucid compliance with their peace agreement permitted Judas Maccabee to attempt to dislodge the Emperor's troops from a stronghold facing Jerusalem, but the regent-general, rushing from Antioch, defeated him soundly.
This was the state-of-play when Judas Maccabeus decided that a treaty with the Roman Senate might be to his advantage. The Senate certainly saw that it was to their liking.
Some people debate whether the treaty was actually formally signed by both parties. Insofar as it is looked at as a Zealot misstep that ultimately invited Rome into Judea and led to the destruction of the Temple, this might be viewed as an important question. (Frankly, it is likely that Rome would have ended up in Judea, with or without the Maccabees, simply due to Roman politics and economics. However, from the perspective of Abraham's covenant with God, these events can take on other significance.) From a more worldly view, the treaty was nowhere near as significant as the escape from Rome in 161 of the son of the Emperor whose brother usurped succession. In the eyes of many, THIS was the rightful Emperor, and that changed everything for the Seleucids and their vassals, including the Jews. His return prompted the execution of both the boy-Emperor and his regent-general. The rightful Emperor then busied himself with an empire collapsing in on top of him, and a civil war in Antioch. The power of the Seleucid Empire never recovered. As it disintegrated, the Roman Senate found itself petitioned by one after another of its former vassal states to intervene in wars and standoffs involving Roman mercantile interests along the Mediterranean littoral. Rome became more and more deeply involved in the Levant as a result, including the administration of Judea. That was NOT the result of a 161 agreement with Judas Maccabeus.