As President Donald Trump withdrew the United States from the Iran nuclear deal on Tuesday, Israel put itself on “high alert.” And there’s a scary reason why: It’s possible Iran will retaliate against Israel with military force after Trump’s decision.

Before Trump’s Iran deal announcement, news broke that Israel had moved to safeguard the Golan Heights — the territory between northeastern Israel and southwestern Syria — to defend against a possible Iranian attack. (Iran has proxies and troops in Syria to help Bashar al-Assad’s regime stay in power, which puts them uncomfortably close to Israel.) Israel even opened up bomb shelters in the area to keep citizens safe.

Tensions are elevated because Israel and Iran are in the middle of fighting a shadow war in Syria. Israeli jets continually bomb Iranian military locations in Syria and have even killed Iranian troops.

That existing conflict, added to Trump’s Iran deal decision, may explain Israel’s caution right now. “Following the identification of irregular activity of Iranian forces in Syria, the IDF [Israel Defense Forces] has decided to change the civilian protection instructions in the Golan Heights and instructs local authorities to unlock and ready shelters in the area,” an IDF spokesperson told me. “Defense systems have been deployed and IDF troops are on high alert for an attack.”

It’s unclear if there will actually be an Iranian attack. It’s possible Israel is just taking some serious precautions against some reports of odd Iranian movements in Syria. However, some Iranian lawmakers did send a letter to Supreme Leader Ali Khamenei asking for a strong response to Trump’s withdrawal from the Iran nuclear deal.

It goes to show that Trump’s decision has far-reaching consequences — and could make relations between Israel and Iran much, much worse.