I hate to break it to you, but the Democrats don’t see the Iranians as a threat. Obama gave them wads of cash and Biden really wants to reinstate the nuclear deal that leaves them free to get the bomb. Nor do they see China as a threat, because they are paid not to. North Korea isn’t a threat to anyone except its immediate neighbors, and short of a regime change invasion, there’s not much we can do if they decide to strike first.
Your argument is just a reduction of the “attack them there so we don’t have to fight them here” argument from the early 2000s. Sounds great in theory, but in practice it only enriches contractors and politicians of the defense Only this time we’re not fighting third worlders, we’re fighting a country with the second largest number of nuclear weapons that won’t hesitate to take over half of Europe if things get bad enough.