"The soldier doesn't get to decide that it's immoral to carpet bomb El Salvador; they've already agreed to kill whoever their leadership tells them to."
Not sure why this is getting down-voted. One of the greatest stories of courage and principled behaviour I have heard of is of the US helicopter pilot Hugh Thompson Jr. who intervened in the My Lai massacre, saving children and civilians. He had to interpose his helicopter between the murdering US troops and the civilians. He ordered his crew to shoot US soldiers if they did not stop massacring civilians. His actions were eventually recognized, but at the time he was vilified by US politicians, and many citizens.
People always have a choice. They can refuse.