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Whether or not the Israeli action is justified, the more pertinent question is will it be effective? (As evidenced in the posts here, anyone can come up with a justification for their position.) By effective, I mean increasing the likelihood of longterm peace and stability and acceptance of Israel.
Military dominance by a superior force guarantees continuation of asymmetric warfare by the weaker side. It also fuels radicalism which drowns out moderate voices. Were I Israel, I'd be extremely concerned about the spread of nuclear know-how (Iran, Libya, Syria, Pakistan). That genie is out of the bottle and only spreading.
The only longterm chance for peace I see is an integrated single state. When Iran, Hezbollah, Syria get the bomb, they would have a much harder time nuking a country with a mixed Israeli-Palestinian population.
Hamas was elected because the only other option was the grossly incompetent and corrupt Fatah. A recent poll shows 72% of Gazans desire peace with Israel:
http://www.ynet.co.il/english/articles/0,7340,L-3456145,00.html
There is potential for a peace. We just need courage on all sides to confront militarism. International support for nonviolent Palestinian parties and an end to the Israeli blockade would be a good start.