The US was under no obligation to fight Great Britain's political battles for them.
Lord Mountbatten was a war hero killed by cowardly terrorist scum. Are you saying every world war 2 vet deserves to be blown up as they lived by the sword?
To a very limited extent, I agree with you. In a war situation Earl Mountbatten could be regarded as a legitimate target. He was a senior politician, and a senior member of the British defence forces.
But the two children who died with him - his grandson, and a 15 year old boy who'd been hired to work the boat - were not legitimate targets. Nor was the 83 year old woman who was killed.
The IRA bombed a boat with three children on board and killed two of them. This was not collateral damage - the Earl would not have been hard to kill on his own - but a deliberate act of terror.