Since the world's most powerful states have been sending undercover agents to their adversary states for millennia, that is hardly a (new) reason to take action

Since the world's most powerful states have been sending undercover agents to their adversary states for millennia, that is hardly a (new) reason to take action against them. Both states have plenty of tools to simply kill every identified agent of the other side, so how far they go in their actions is always a policy decision, balancing goals with retaliation. Plausible deniability helps, at least for awhile.