My Standard Caveat: I’m not a lawyer, nor do I play one on this website. This is the information as best I understand it, but you may wish to consult with a legal professional for your specific needs.
Let’s start with a very important point: self-defense has a legal definition in most places, and if your actions stray from the perimeters of that definition, you could land in legal trouble. Now, the good news is that your right to self-defense is nearly universally recognized—most cities/states/nations have laws permitting you to take actions to protect yourself that might otherwise be against the law.
But the Devil is in the details, as always. In the US alone, there are a myriad different laws governing self-defense, depending on the jurisdiction in which you find yourself. However, there’s a key underlying principle, embodied in the definition of self-defense offered at Law.com:
self-defense n. the use of reasonable force to protect oneself or members of the family from bodily harm from the attack of an aggressor, if the defender has reason to believe he/she/they is/are in danger.
The definition continues—and you should read it in full—but the first sentence is packed with meaning. Reasonable force is just that—the force used in self-defense must be what is necessary to end or prevent the threat, and no more. Hitting an attacker could be appropriate; chasing them with a kitchen knife as they try to escape crosses the line into assault. Likewise, the danger presented needs to be physical, hence bodily harm. Self-defense law applies when you’ve done nothing to provoke the incident, hence the phrase attack of an aggressor. Starting a fight—even verbally—and then responding with physical force can undermine your claim of self-defense.
The phrase reason to believe needs its own, albeit brief, paragraph. Like a great deal of common law, self-defense law operates on the “reasonable person” model—i.e., how would a “fair minded and informed observer” of reasonable bent have interpreted the situation. The upshot is that what may appear reasonable to you in the heat of the moment may be less so to someone outside the situation, so plan and practice accordingly.
Just as laws defining self-defense vary, so do the regulations around the tools that one may use or carry to do so. The restrictions on items as simple as sticks, kubotans, and pepper spray can be wildly different depending on the jurisdiction in which you find yourself. You’re responsible for knowing the relevant law, so please do your research and consult with a professional if you’ll be traveling. This is doubly true if you’ll be flying. The TSA’s rules are obtuse and the variety of items they confiscate is both dazzling and evidence of uneven enforcement. Something as simple as a kubotan or tactical pen may get you into trouble there.
There’s a lot to take into account here, and unfortunately if you’re ever in a self-defense situation you’ll be required to to manage all of this information in addition to executing the correct response in a hurry and under pressure. Thinking about it in advance and planning/practicing accordingly can go a long way toward a successful outcome.