He would have been drafted upon graduating from college, but he pulled strings to avoid it. He had won the right to be a Rhodes scholar but needed to get out of the draft. He did this by agreeing to return and go to the University of Arkansas law school and jjoin ROTC there. Making this agreement - a ROTC colonel was able to get him out of the draft so he could go to England. When the draft was changed to the lottery, Clinton broke that agreement. Now, all of that made sense because he at that time 1969/1970 was completely against the war.
This became a problem in 1992, because he lied about it - giving as a first response that he didn't remember if he ever got a draft notice. As a girl then, I never got one, but I can tell you I know receiving that letter was not easily forgotten by any guy I knew. He spoke of getting a low lottery number - but it then came out that he was vulnerable before the draft. He said he pulled no strings when he did. He was then caught when the letter came out. The surrogate who helped him the most in getting past this - John Kerry, war hero.
In the primaries, Kerry defended Clinton's choice and begged fellow Democrats not to re-open the wounds caused by Vietnam. Kerry ignored the lies, speaking just on the decision not to serve - the speech is beautiful as was Kerry's motivation - he was not for Clinton, he was a friend of Tsongus and Kerrey. (link to speech -
http://journals.democraticunderground.com/karynnj/18 )
Where he had a problem was that in deciding not to honor the agreement he wrote a letter that was, to some degree, a slap in the face of the man who helped him avoid the draft. One line spoke of how some loathed the military.