Answers to this question may, of course, be subject to great debate and disagreement. Here are a few possible answers:
1. The American people are unlikely to support a war for many years unless the original reasons for the war and the long-term purposes of the war are very clear.
2. The American people are unlikely to support a war for many years unless the war seems to be accomplishing its objectives successfully and relatively quickly.
3. The American people are unlikely to support a war for many years if the war is designed to support a regime that can't seem to muster strong support from its own people.
4. Use of a draft to find soldiers to fight a war is likely to be very unpopular if the war fails to command widespread support.
5. Any draft should affect all segments of the population equally.
In the words of Donald M. Goldstein in the introduction to this book The Vetnam War:
The Vietnam War was arguably the most traumatic experience for the United States in the twentieth century. That is indeed a grim distinction in a span that included two world wars, the assassinations of two presidents and the resignation of another, the Great Depression, the Cold War, racial unrest, and the drug and crime waves.