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One has the right to self-defense if one is not oneself guilty of aggression. So, for example, the Soviet Union could not invoke self-defense when its occupation troops in Afghanistan were attacked by Afghan mujahideen. Instead, it ought to have withdrawn its troops. Likewise, the Israeli occupation of the Palestinian territories is illegal and unjust and Israel can't claim self-defense when Palestinians struggle by legitimate means to end the occupation. The proper Israeli response to such Palestinian actions is not self-defense, but full withdrawal from the occupied territories.
The situation with Lebanon is different; whereas in Palestine, Israel was engaged in an ongoing aggression, in Lebanon the Israeli violations of Lebanese rights prior to July 12, 2006, were far less substantial, and less immediate.
But even when a country's own prior acts aren't contributory causes of an attack, international law places various limitations on the right of self-defense to that attack.
One limitation is that the right of self-defense is meant to give nations the right to take measures to repel an armed attack until the UN Security Council can act to stop the aggression. If an enemy's tanks are hurtling toward your capital city, any delay in responding would mean further losses and further harm. In the case of the Hezbollah raid across the Israeli border on July 12, 2006, the act of aggression took place and was over; it was not an ongoing aggression to which any delay in responding would have meant additional harm to Israel. Once the immediate danger is over, international law requires that victims of aggression bring their cases to the Security Council for action.
Of course, the Security Council is not always able to act. But the main obstacle to Security Council action has generally been the veto wielded by Washington on behalf of Israel.
A second requirement of international law is that acts taken in self-defense must be proportionate to the offense.
Stephen Shalom (ZNet)