In the first in Stance, the Arabs were in an officialnstate of war with Israel since 1948. Secondly, Egypt injected her army divisions into Sinai and right up to the Israeli border. This was a breach of the cease fire agreement after the 1956 war. Thirdly, Egypt kicked out the UN peacekeeping force on the Israeli/Egyptian border.which was another breach of the agreement and finally, Egypt blocked the straits of Tiran, another breach. Each one of these breaches was a caucus belli for the resumption of hostilities as they were acts of war, breaching he ceasefire. Furthermore, these actions combined with the bellicose statements of Arab leaders threatening the destruction of Israel, gave her the right to take per emotive action in accordance with International law. So the Arabs committed the first acts of war which gave Israel he right to commence hostilities.
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from the article: Six Day War breaks out, this time on Twitter