Well, first off, Troy at this time was considered equal in power to the Mycaeneans (Sp?), so with their defeat we should see at least 90 years of Trojan dominted Greece, Asia Minor, and the Caspian.
First of all, Troy was NOT considered equal in power to the Mycenaeans. Troy was normally a sub-kingdom of the Hittite Empire, while the King of Ahhiyawa (which most scholars consider to be an overlordship of Mycenae over the other Greek cities) was considered a "Great King" on the level of the King of Hatti, the King of Babylon, the Pharoah of Egypt, and the King of Assyria. Troy was NEVER at or anywhere near that level. The idea that they would dominate anything beyond their corner of Asia Minor is pretty ludicrous.
The reason why the Greeks had difficulty taking the city was 1) the Trojans likely had help from the Hittites, and 2) siege technology didn't exist. Even something so basic as the battering ram had not been developed by the time of the siege.
I say 90 years because in 1100 BC (With the war being in 1190) the first invasions of the Dorians and the Sea People began.
The 1190 date is in dispute. Troy was destroyed twice within a relatively short time...at the end of the archaelogical level known as Troy VI, and at the end of the level known as Troy VIIa. The Troy VI destruction is dated to roughly 1250 BC, and Troy VIIa's to about 1190 BC. While some scholars attribute the 1250 BC destruction to an earthquake, there is reason to believe that this, rather than the town destroyed in the 1190 BC destruction, was the city of the Homeric Trojan War.
--Troy VI greatly resembles the rich and powerful city described by Homer, whereas Troy VIIa was a much poorer and less imposing place.
--The Mycenaean kingdoms were at the height of their power in 1250 BC. By 1190, most of the Mycenaean cities were in ruins, destroyed by the Dorians, which begs the question of how they could have at the same time fought an extended war against Troy.
--And indeed, the earthquake theory fits neatly into this scenario, because of the legend of the Trojan Horse. The horse was the symbol of Poseidon, the Sea god who also caused earthquakes. So the Trojan Horse story may be a distant folk memory and/or metaphor for an earthquake which breached the city walls, allowing the Greeks to come in and slaughter the inhabitants.
So, most likely, the Mycenaeans destroyed the city in 1250 BC, it never fully recovered, and then the Sea Peoples (or the ancestors of the Phrygians, who were crossing over into Asia Minor from Thrace at this time) destroyed it again in 1190 BC.