I disagree with this point. Due to military conscription in WWI, WW2 and other wars, intelligent and unintelligent men within the selected age range were just as likely to end up in mortal combat. Possession of an above average intelligence wouldn't have done much to stop you from being mowed down by a German machine gunner at the Battle of the Somme and if you had "broken the line" or refused to go "over the top" then you would have been shot for cowardice. That's just one example of a 20th century war scenario, but there are plenty of equally dangerous and arbitrary ones.
Intelligent men may also have felt morally obliged to volunteer for active service during WWI and WWII, in order to preserve their freedom and way of life. One case is that of the brilliant physicist H.G.J. Moseley who discovered that each element has a unique atomic number. He volunteered for military service during World War I and became a signal officer in the British army. He was killed in action in Gallipoli in 1915.
The effect of wars in the 20th century was probably to make the population less fit. This was because the physically fit males were more likely to be killed in the war before they could reproduce, (since they were drafted first), whereas those with genetic disorders, physical and mental disabilities etc, were not drafted and so were never in harm's way. In wars before the advent of machine guns, poison gas, aerial bombing, tanks, artillery etc, the evolutionary effect of war might have been to select for the more physically agile/powerful and intelligent individuals. But in most 20th century wars those traits would probably have been less helpful to an individual soldier's survival than luck.
Before WWI, French men were statistically the tallest in Europe. Shorter, less healthy men were the last to be drafted and therefore less likely to die. That's probably why Frenchmen are so short and mousy these days.