In countries with mandatory Military service, What happens if individuals physically are unable to serve?
Well, it varies by country. Some countries might excuse a person from service if their condition was significant enough. Other countries just find the person something else to do in the military; only about 10% of any military consists of primary, full-time combat forces anyway. There are plenty of other jobs to do in any military. As an example, if they were wheelchair bound they could do desk/computer work.