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In the escalating confrontation between Israel and Iran, Israel's defense minister called on Syria's President Bashar Assad on Friday to rid his country of Iranian forces based there — warning their presence will only cause more trouble to the already war-ravaged country.

Avigdor Lieberman's comments were followed by threats from an Iranian cleric that Tel Aviv or Haifa would be in danger if Israel did "anything foolish."

The war of words followed the worst face-off to date between Israel and Iran. Israel fired dozens of missiles at what it said were Iranian positions in Syria on Thursday, hours after it said its forces in the occupied Golan Heights were targeted by Iranian rockets.

The brief but intense confrontation raised fears the region may be sliding into an unprecedented direct conflict between the two archenemies.

Here is a look at why the two countries are at each's other throats and why Syria could be the ground for a showdown.

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WHY SYRIA?

Iran sent massive military help to its ally, Syrian President Bashar Assad, to rescue his rule against armed rebellion during that country's 7-year-old civil war. With the war winding down in favor of Assad, Israel — which saw him as the lesser of two evils compared to Islamic hard-liners among rebels — is now finding that his victory has brought Iran closer to its borders.

Israel has increasingly warned that it sees Iranian influence in Syria as a threat, pointing to Iran's military presence inside the country as well as that of Iranian-backed militiamen. Israeli officials have said that 80,000 Shiite fighters in Syria are under Iranian control, including forces of Lebanon's Hezbollah and Iraqi and Afghan fighters.

Iranian officials and their allies have spoken of securing a corridor from Iran to Lebanon, through Syria and Iraq. Israel fears that will allow Iran to more easily transfer weapons to Hezbollah, Lebanon's powerful Shiite guerrilla force, and reinforce the militant group's influence over the region. During the civil war, Israel is believed to have carried out hundreds of strikes in Syria, mainly hitting weapons shipments.

Tension has been building. An airstrike on a military base in Syria last month, which Iran and Russia blamed on Israel, killed seven Iranians. In February, Israel shot down an Iranian drone that entered its airspace, triggering a clash in which an Israeli warplane crashed after being struck by Syrian anti-aircraft fire.

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WHAT DOES THE NUCLEAR DEAL HAVE TO DO WITH RISING TENSIONS?

Israeli Prime Minister Benjamin Netanyahu was a sharp critic of the Iran nuclear deal. Now, with President Donald Trump's decision to withdraw from it, Netanyahu may be emboldened to pursue his confrontation with Iran.

Tehran, meanwhile, is under pressure from the U.S. and Western allies to negotiate a new deal, one that goes beyond restricting the nuclear program to curb Iran's military power in the region. Iranian officials have rejected any new accord.

Tehran has not completely walked away from the nuclear deal, since the Europeans are still participating. But if it completely collapses and the U.S. imposes heavy new sanctions on Iran, prospects for major fallout with Israel are higher.

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HAVE ISRAEL AND IRAN ALWAYS BEEN MORTAL ENEMIES?

During the rule of Shah Mohammed Reza Pahlavi, Iran had close relations with Israel starting in the 1950s, including diplomatic representations and direct flights. The two countries were the main allies of the United States in the region, and Iranian oil was shipped to Israel during the 1973 war.

But the 1979 Islamic Revolution ousting the shah ended that. The revolution's leader, Ayatollah Khomeini, declared Israel an "enemy of Islam" and cut all ties. Hostile rhetoric has escalated over the decades since. Israel considers the government in Iran an existential threat.

But unlike Israel and its Arab neighbors, the two countries have never had a direct confrontation. Instead, Israel has fought Iran's ally, Hezbollah, the last time in a 2006 war that saw massive destruction in southern Lebanon and concentrated rocket fire on Israeli cities.

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IS THE MIDEAST SLIDING TO AN ALL-ENCOMPASSING, RUINOUS WAR?

The reported Iranian attack on Israeli positions in the Golan and Israel's bombardment of suspected Iranian posts in Syria appeared to be warnings by each side that it is willing to respond — but not necessarily that they want to plunge into war.

But if they do escalate, the region could face one of its worst cross-border conflicts in decades, one that could potentially drag in the United States, a major ally of Israel, and Russia, which is Syria's mightiest ally.

Although Iran may not be a match for Israel's military power, it has a variety of allies and ways to hit back if corned by the U.S., Israel and Saudi Arabia, Iran's regional rival.

Iran has an ally in Hamas, the Palestinian militant group in Gaza. In Lebanon, Hezbollah would stand ready to support its patron Iran. In Iraq, Iran sponsors a range of Shiite militias and has close ties to the political leadership.

In Yemen, the war in its fourth year is seen as a proxy between Saudi Arabia and Iran and can be a place for Tehran to retaliate and increase pressure. Saudi Arabia already accuses Iran of providing missiles that Yemeni rebels have fired toward Riyadh.