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Israel has often had hostile relations with its Muslim neighbors. But right now its greatest enemy may be Iran, which has one of the most powerful militaries in the region and has for years been openly hostile toward the very existence of Israel.

The situation may only be getting worse, with Iran seemingly on the rise since the Obama administration hatched a deal with the country that lifted international sanctions and gave the Islamic Republic approximately $100 billion in frozen assets.

Israel’s relations with Iran have changed since the Jewish state’s founding in 1948. Up until 1979, the two countries had relatively close ties. With Israel sometimes at war with its Arab neighbors, the non-Arab Iran was an important ally.

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But in 1979, the Islamic Revolution, with its leader Ayatollah Khomeini, took over in Iran, and Israel became the enemy.  Iran closed the Israeli embassy and cut off diplomatic relations.  It referred to Israel as “The Little Satan.” (“The Great Satan” was reserved for the United States.)

Still, even with officially hostile relations, Israel supported Iran in its war against Iraq from 1980 to 1988, selling it arms and even destroying Iraq’s Osirak nuclear reactor in a surprise air strike in 1981.

Then, in 1989, the Grand Ayatollah Khamenei assumed power and relations took a turn for the worse.  Khamenei referred to Israel as a “cancerous tumor in the heart of the Islamic world.”

While Israel and Iran have not had open military clashes, they have battled each other through proxy fights.  In the past few decades, Iran has sent weapons and hundreds of millions of dollars to anti-Israel terrorist groups, in particular Hezbollah and Hamas. The Israeli military has spent much time preventing or responding to attacks from these groups.

There have also long been rumors that Israel might strike one of Iran’s nuclear facilities, but so far this hasn’t happened.

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Nevertheless, Israel has fought against Iran in more surreptitious ways, particularly regarding attempts to stop or slow down the nation’s development of nuclear weapons.

For instance, in 2008 to 2010, the malicious “Stuxnet” computer worm, developed by Israel and the United States, severely disrupted Iran’s nuclear program.

Also, starting in 2010, a number of Iran’s nuclear scientists were assassinated, and it’s generally believed Israel and its intelligence agency Mossad were behind the killings.

Today, some fear if Iran becomes more powerful—and especially if it attains nuclear weapons—clashes may escalate.

At present, in addition to bad blood between Iran and Israel, the Islamic world itself is split, with centuries-old Islamic denominational strife coming to the fore.  Iran is the heart of the minority Shia denomination, while most of the Middle East is Sunni, with its center arguably being in Saudi Arabia, home of Mecca, Islam’s holiest city.

Iran and Saudi Arabia’s relations have become increasingly strained, and many Arab nations fear the rise in influence of Iran. This may cause them to look toward Israel, with its powerful military, as a helpful entity that contains Iran’s power and perhaps even stabilizes the region.