The conflict, the most intense fighting between the two countries in decades, has been met in the United States with feelings of “frustration and helplessness,” as well as heartbreak.

DUBAI — Israel unleashed airstrikes across Iran for a third day Sunday and threatened even greater force as some Iranian missiles evaded Israeli air defenses to strike buildings in the heart of the country. Planned talks on Iran’s nuclear program, which could provide an offramp, were canceled.

The United States is shifting military resources in the Middle East in response to Israel’s strikes on Iran and a possible retaliatory attack by Tehran.

The strikes are a sign of Israel’s shift away from restraint, containment and short wars, and toward using force to project power across the Middle East.