The Reason Why Israel Will Invade Gaza
For Hamas, the terrorist organization inside Gaza that the PLO does not control (so they claim), it is a prestige weapon in limited supply: the Iranian Fajr-5 rocket. It carries a 200 lb. warhead and has a range of 45 miles, enough to terrorize Tel Aviv and Jerusalem, cities that seldom attacked. While the rocket is not that accurate and could hit anywhere within a mile of its intended impact, this is something Israel cannot tolerate coming from Gaza (although, the rocket is also in the hands of Hezbollah in Lebanon).
The reason for Cast Lead 1 and soon, 2, operations with Israeli military entering Gaza is to hunt and find the remaining Fajr-5 rockets that have not yet been destroyed from the air. The IDF airforce has done a great job so far of destroying about 20 of them in known or suspected areas and underground bunkers, but Hamas still has some. Probably not more than 10. Thus, the only way to find them is to enter Gaza and search for them, destroy them and leave. To think Israel wants or needs Gaza now is laughable, it would be more of problem than its worth. Israel's only target is Hamas and their weapons.
But Israel also seems to be subtly saying that our "red line" is Hamas having this rocket. Israel seems to be willing to tolerate the shorter range missiles raining down on small towns. It seems that rockets landing in Israel's cities is not acceptable, yet, landing in smaller towns is fine.
The Iranian's claim they did not supply Hamas with the rocket, okay, but they did so to Hezbollah in Lebanon, which then smuggled the missiles into Gaza with the tacit acceptance of Egypt through the Sinai border that Egypt regulates. That is the only way they get in and reassembled.