Intelligence should be positively correlated with survival in war; on the other hand, intelligence may be correlated with the amount of risk shouldered, as more intelligent men may be preferred to carry out complex high-risk mission. So, it's not clear whether smarter-than-average or not tend to survive wars.
This study shows that among Scottish Army participants of WWII, survivors were less intelligent than non-survivors, but also that non-participants in the Army were more intelligent than participants. Of course, this latter finding may be due to non-Army men being e.g. in the Navy, for which no data exist.
Certainly this data shows some relationship between IQ and military participation/survival, although the effect does not seem to be very pronounced.
The WWII experience was probably one of near-universal participation by the combat-age male population of the countries involved, and it would be interesting to see comparable data from volunteer Armies, such as the current US Army, where IQ data probably exist.
Childhood IQ and in-service mortality in Scottish Army personnel during World War II
Janie Corley et al.
The Scottish Mental Survey of 1932 (SMS1932) provides a record of intelligence test scores for almost a complete year-of-birth group of children born in 1921. By linking UK Army personnel records, the Scottish National War Memorial data, and the SMS1932 dataset it was possible to examine the effect of childhood intelligence scores on wartime military service mortality in males. There were 491 matches between World War II (WWII) Scottish Army fatalities and the SMS1932 database; 470 (96%) had an age 11 mental ability score recorded. The mean (S.D.) age 11 IQ score of those who died on active service in WWII was 100.78 (15.56), compared with 97.42 (14.87) for male Army survivors (p less than 0.0001; Cohen's d = 0.22). Men who took part in the SMS1932 and who were not found in the Army database had a higher mean score (100.45, S.D. =14.97) than those men who had been in the Army, regardless of whether they died or survived (mean IQ = 97.66, S.D. = 14.94; p less than 0.0001; Cohen's d = 0.19). Male soldiers with a higher childhood IQ had a slightly increased risk of dying during active service in WWII. Men who did not join the Army had a higher IQ than men who did. Further research in this area should consider naval and air force personnel records in order to examine more fully the complex relationship between IQ and survival expectancy during active service in WWII.