Primarily, the Iranian Revolution in 1979. The Shah, who had been the ruler until then in Iran, had ruled with an autocratic fist. This meant that he, who was Western-aligned, was also able to cooperate to some extent with the increasingly Western-aligned state of Israel. This cooperation also provided him with an outlet for oil, which Israel desperately wanted to ship in. Israel also provided a pathway for a pipeline that transported Iranian oil to Europe.

The Revolution changed that. The new government was staunchly anti-West, and referred to Israel as "Little Satan" regularly. Nevertheless, relations taking a hit did not actually kill relations. During the Iran-Iraq war, Israel thought there was still an opportunity to operate what it called the "alliance of the periphery", an idea formulated by its first Prime Minister, David Ben-Gurion. The idea was to ally with states on the outskirts of the Arab world, which were not Arab themselves, as natural allies against the Arab world. This led to Israeli outreach to countries like Turkey, Iran, and Ethiopia. Israel attempted to assist Iran during the Iran-Iraq war, supplying weapons (this figured into the later Iran-Contra scandal in the US, actually). Following the war's end, however, the Iranian regime chose to go back on the offensive rhetorically against Israel, and relations soured from there. Nevertheless, there was a brief period during which Israeli assistance might've meant a continuing alliance with Iran, despite the Iranian Revolution.