Lets look at this from a big picture perspective. Here's a map of the area:
The US is in Iraq and Israel is moving into Lebanon. This surrounds and isolates Syria.
There has been a lot of pressure on Syria since the US moved into Iraq. The US had several battles with insurgents near the Syrian border and probably crossed the border a few times chasing them. The US has also warned Syria about helping "terrorists."
Syria was recently forced out of Lebanon. Israeli war planes flew over the Syrian leader's home a short time ago and created a couple of sonic booms in the process. Israel has made many threats toward Syria in the past year.
Syria is made up of Shiite Muslims. So is Iran. Hezbollah is Shiite and is supported by Syria and Iran. The majority in Iraq is Shiite. Iran controls the Shiites in Iraq and is using this as a bargaining chip against the US. Iran is also using its influence over Hezbollah as an indirect threat to the US. Iran has recently made very direct threats against Israel. Syria also has influence over Shiite Iraq, Hezbollah and Lebanon, and is a natural ally of Iran.
Weaken Syria and you weaken Hezbollah, Shiite Iraq, and Iran. If you look at it this way the Lebanon invasion is a positioning move and Israel will use the excuse to occupy Lebanon and turn up the heat on Syria.
