Families and loved ones say goodbye to troops deployed to the Middle East

Iran's vow to strike the US military in retaliation for Soleimani's killing presents the Pentagon with a massive force protection problem.

A military threat: In an exclusive CNN interview on Sunday, Maj. Gen. Hossein Dehghan -- the military adviser to Iran's Supreme Leader, Ayatollah Ali Khamenei -- said that Tehran's response to the US drone strike would be to hit back directly on American "military sites."

What this means for the US: From Singapore to Djibouti and Bahrain to Brazil, today the US operates about 800 military bases and logistical facilities outside its sovereign territory -- more than any other nation.

And every US installation -- even every soldier, sailor, airman or Marine -- could be a target. Presumably that could also include US Navy ships at sea, US Air Force planes at airports or in the world's skies, or even US troops enjoying themselves off base and off duty.

"There are many ways to hit US personnel, and you can't protect them all," said Carl Schuster, a former director of operations at the US Pacific Command's Joint Intelligence Center.

Proxy threat: The threat could come from Iranian forces themselves -- or proxies deeply tied to Tehran, like the Lebanese militant group Hezbollah, which has reach across the Middle East and into Africa.

While the US military will be on heightened watch, it's hard to stay on constant alert.

"Tight security tends to impede your operations," Schuster said, citing the need to resupply bases or ships and the movement of personnel in or out. "You can't guard them all the time for a very long period of time."

Read the full analysis here.