Can I be terminated if I was struck by employee and knocked down to the floor and upon gaining control, I defended myself?

In Wisconsin, and many other places, an employee can be terminated for no reason, or any reason. (Except of course for reasons of race, religion, national origin, sexual orientation, etc.) However, if there exists some kind of contract between the employer and the employee (including a union contract), the the employee can only be discharged in accordance with the contract. An employee handbook which includes 'progressive discipline,' or termination only 'for cause,' will often be deemed a contract as well.