The spread of that network means that Iran could rely on allied forces to strike at any number of places across the region while giving Iran itself a way to deny responsibility.

As the United States’ regional allies try to anticipate Iran’s next move, they are shoring up their defenses, keeping their distance from Mr. Trump’s decision to kill General Suleimani and even quietly reaching out to Iran directly to try to avert conflict.

The situation is particularly delicate for countries like Saudi Arabia and the United Arab Emirates that sit directly across the Persian Gulf from Iran and have close ties to the United States. Their wealth and vast oil and commercial infrastructure could be attractive targets for Iranian attacks.