Throughout history one of the things that has terrified leaders is the idea of a large, aimless population of young men with nothing to do.

The Romans tried to give them land to farm when they had to, but generally preferred to expend those young men's lives on battlefield conquests.

Some countries were more than eager to join WWI because of fears of socialist revolution stirring at home. Framing the war as a struggle for homeland pushed many of those revolutionaries towards nationalism.

Hitler rose from a humiliated and economically broken Germany.

I find it more likely that rather than intentionally keeping people poor to drive recruitment, that people are being kept poor by corporate greed and the U.S. armed forces exist as something of a job's program for those with war-like leaning and few prospects to keep them from getting themselves into too much trouble. And I say this as a veteran who spent seven years active duty.

After all, the branches are usually selective enough to keep some of the absolute poorest out still. There's not a lot of latitude for criminal activity, for instance.

Obligatory: this is not indicative of everyone who joins the military or even the majority. There is just a noticeable contingent of violent, aggressive, kind of person who really needs the military structure to keep them from a jail cell.