He wasn't given full immunity. He was given immunity from actions taken "lawfully" or within the confines of his authority. Declaring war without congressional approval is not an executive branch sole decision, it belongs to the members of congress. He can face charges for egregious actions, which at the time of the ruling, the SC said would be to varied to describe all circumstances that could be involved. People often miss that part of the ruling. The SC finally stood up to him the other day when they ruled that he couldn't just claw back unspent funding to states, which this particular incident stemmed from two states suing him. Only congress, they told him, have the power to engage in monetary affairs. They also hit him on the executive order for deadlines states sit for ballots to be received after election day. Once again, they told him, only congress can change the rules for federal elections. Many mistook this as meaning states had absolute power over elections, they don't, they can make their own rules for state elections, but they have to follow the rules sat by congress for federal elections. The ruling was just telling him, as Musk would say, "how many steps he can take back from himself" considering he could just order rule changes delegated to congress.