The invasion of Iraq in 2003 wasn’t legitimate, the Gulf War, however, was. The United Nations say that each country has the right to defend itself if invaded or feels the threat to be invaded, attacked. Under that rule it was decided that US-led coalition can free Kuwait of Iraqi forces.
As for the Gaza strip, it is de-facto a country, but de-jure, it’s an ungoverned piece of land. A piece of once promised land. This conflict doesn’t reach back to 1948, when Isreal emerged as a country, but back to the times of Moses and the pilgrimage of his people to the promised land. Then, the Palestinians inhabited the land. In 1948, Jews returned. Different religions couldn’t co-exits in one country, Israel, which caused the Palestinian resistance. It’s not the first time blood was shed and it won’t finish soon.
Palestinians want their own state. Israelis won’t let them. It’s happened before. Kosovo and Serbia, South Ossetia and Georgia.