Militant attacks on security forces and unrest in Iran’s main oil hub threaten the country’s economic engine as the regime struggles to cope with tighter U.S. sanctions, a falling currency and growing regional tensions.

On Saturday, gunmen killed more than 25 people at a military parade of the elite Islamic Revolutionary Guard Corps in the southwestern province of Khuzestan. It was the third attack on security forces in the province in the past three years; five people died in the prior attacks, according to the government.

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