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Israel is the legal successor state to the British Mandate for Palestine as it was the only state established within its borders. That gives Israel a legal claim under international law to the whole territory.
Palestinians have a claim to the land because the population has lived in the land for a long time and can establish a state under the right to self determination.
These are competing claims.
They don’t need to have been sovereign before to be able to exercise self-determination.
Palestinians don’t have a claim to the State of Israel. They have a right to form their own political entity.
The rocket attacks were justified by pointing to the Israeli invasion of southern Lebanon in 1978.
The rocket attacks on Israel today are a couple of decades too late to use this justification.
The reason for the Israeli invasion into Lebanon was a series of attacks by organized militants on Israel from Lebanese territory.