It has been hypothesized that those who entered military service in the pre-September 11, 2001 era might have expectations incongruent with their subsequent experiences, increasing the risk for posttraumatic stress disorder (PTSD) or other mental disorders. A subset of Millennium Cohort Study participants who joined the military during 1995–1999 was selected and compared with a subset of members who joined the military in 2002 or later. Outcomes included new-onset symptoms of PTSD, depression, panic/anxiety, and alcohol-related problems. Multivariable methods adjusted for differences in demographic and military characteristics. More than 11,000 cohort members were included in the analyses. Those who entered service in the pre-September 11 era had lower odds of new-onset PTSD symptoms (odds ratio [OR] 0.74, 95% CI [0.59, 0.93]) compared with the post-September 11 cohort. There were no statistically significant differences in rates of new-onset symptoms of depression, panic/anxiety, or alcohol-related problems between the groups. The cohort who entered military service in the pre-September 11 era did not experience higher rates of new-onset mental health challenges compared with the cohort who entered service after September 11, 2001. Findings support the concept that the experience of war, and resulting psychological morbidity, is not a function of incongruent expectations.