Excerpt from Wikipedia on the Macedonian Wars:
Second Macedonian war (200 to 196 BC)
The past century had seen the Greek world dominated by the three primary successor kingdoms of Alexander the Great's empire: Ptolemaic Egypt, Macedonia and the Seleucid Empire. The imperial ambitions of the Seleucids after 230 BC were particularly destabilizing. The Seleucids set out to conquer Egypt, and Egypt responded through a major mobilization campaign. This campaign led to military victory against Seleucid incursions, but in 205 BC when Ptolemy IV was succeeded by the five-year-old Ptolemy V (or rather, by his regents), the newly armed Egyptians turned against each other. The result was a major civil war between north and south. Seeing that all of Egypt could now be conquered easily, the Macedonians and Seleucids forged an alliance to conquer and divide Egypt between themselves.
This represented the most significant threat to the century-old political order that had kept the Greek world in relative stability, and in particular represented a major threat to the smaller Greek kingdoms which had remained independent. As Macedonia and the Seleucid Empire were the problem, and Egypt the cause of the problem, the only place to turn was Rome. This represented a major change, as the Greeks had recently shown little more than contempt towards Rome, and Rome little more than apathy towards Greece. Ambassadors from Pergamon and Rhodes brought evidence before the Roman Senate that Philip V of Macedon and Antiochus III of the Seleucid Empire had signed the non-aggression pact. Although the exact nature of this treaty is unclear, and the exact Roman reason for getting involved despite decades of apathy towards Greece (the relevant passages on this from our primary source, Polybius, have been lost), the Greek delegation was successful. Initially, Rome didn't intend to fight a war against Macedon, but rather to intervene on their behalf diplomatically.
Rome gave Philip an ultimatum that he must cease in his campaigns against Rome's new Greek allies. Doubting Rome's strength (not an unfounded belief given Rome's performance in the First Macedonian War) Philip ignored the request, which surprised the Romans. Believing their honor and reputation on the line, Rome escalated the conflict by sending an army of Romans and Greek allies to force the issue, beginning the Second Macedonian War. Surprisingly (given his recent successes against the Greeks and earlier successes against Rome), Philip's army buckled under the pressure from the Roman-Greek army. Roman troops led by then consul Titus Quinctius Flamininus reached the plain of Thessaly by 198 BC. In 197 BC the Romans decisively defeated Philip at the Battle of Cynoscephalae, and he sued for peace. In the resulting Treaty of Tempea, Philip V was forbidden from interfering with affairs outside his borders, and was required to relinquish his recent Greek conquests. At the Olympiad in 196 BC Rome proclaimed the "Freedom of the Greeks", which constituted Rome's (arguably misguided) new policy towards Greece. This was that Greece was now stable and Rome could completely remove itself from Greek affairs without risking more instability. It seemed that Rome had no further interest in the region, as they withdrew all military forces without even attempting to consolidate any gains, and subsequently returned to their prior apathy even when their Greek allies ignored later Roman requests.
Seleucid War (192 to 188 BC)
With Egypt and Macedonia now weakened, the Seleucid Empire became increasingly aggressive and successful in its attempts to conquer the entire Greek world. When Rome pulled out of Greece at the end of the Second Macedonian War, they (and their allies) thought they had left behind a stable peace. However, by weakening the last remaining check on Seleucid expansion, they left behind the opposite. Now not only did Rome's allies against Philip seek a Roman alliance against the Seleucids, but Philip himself even sought an alliance with Rome. The situation was made worse by the fact that Hannibal was now a chief military advisor to the Seleucid emperor, and the two were believed to be planning for an outright conquest not just of Greece, but of Rome also. The Seleucids were much stronger than the Macedonians had ever been, given that they controlled much of the former Persian Empire, and by this point had almost entirely reassembled Alexander the Great's former empire. Fearing the worst, the Romans began a major mobilization, all but pulling out of recently pacified Spain and Gaul. They even established a major garrison in Sicily in case the Seleucids ever got to Italy. This fear was shared by Rome's Greek allies, who had largely ignored Rome in the years after the Second Macedonian War, but now followed Rome again for the first time since that war. A major Roman-Greek force was mobilized under the command of the great hero of the Second Punic War, Scipio Africanus, and set out for Greece, beginning the Roman-Syrian War. After initial fighting that revealed serious Seleucid weaknesses, the Seleucids tried to turn the Roman strength against them at the Battle of Thermopylae (as they believed the 300 Spartans had done centuries earlier to the mighty Persian Empire). Like the Spartans, the Seleucids lost the battle, and were forced to evacuate Greece. The Romans pursued the Seleucids by crossing the Hellespont, which marked the first time a Roman army had ever entered Asia. The decisive engagement was fought at the Battle of Magnesia, resulting in a complete Roman victory. The Seleucids sued for peace, and Rome forced them to give up their recent Greek conquests. Though they still controlled a great deal of territory, this defeat marked the beginning of the end of their empire, as they were to begin facing increasingly aggressive subjects in the east (the Parthians) and the west (the Greeks), as well as Judea in the South. Their empire disintegrated into a rump over the course of the next century, when it was eclipsed by Pontus. Following Magnesia, Rome pulled out of Greece again, assuming (or hoping) that the lack of a major Greek power would ensure a stable peace, though it did the opposite.
These two campaigns were show-cased in an old SPI board-game (The Conquerors). The Seleucid war is a lot of fun because it has cameo appearences by some of your favorite characters from the 2nd Punic War.
The 3rd Punic War would round out the saga and make a good introductory scenario (as would the Third Macedonia War).