It is difficult to establish the precise causal relationship between military service and Republican orientation. It may be that service in the military per se socializes an individual in certain ways that in turn lead to a more Republican viewpoint — either at the time or in later years. On the other hand, there may be a selection factor at work, such that individuals already disproportionately Republican in orientation are more likely to join the military, meaning that the causal factor predates actual military service.
The latter explanation seems more reasonable for the younger age cohorts considered in this research. For the most part, Americans who are now aged 55 and under, as noted, volunteered to serve rather than having been drafted. Under these conditions, a reasonable hypothesis seems to be that more conservative/more Republican persons would be disproportionately represented in the ranks of volunteers, suggesting that the major reason for the observed veteran/nonveteran political difference lies in the backgrounds of those who choose to serve.