How far would the US go to defend Israel?

The US has pledged its unwavering support to Israel and backed that up with military aid. But with the scars of past entanglements in the region still being felt, where is the limit to US involvement?

In his first reaction to the attack on Israel by Hamas, President Biden made clear whose side he's on: "The United States has Israel's back," he said.

"To anyone thinking of taking advantage of the situation, I have one word: Don't," he added.

This warning was clearly aimed at Iran and its allies.

US...