Chris Blattman, the research economist who left teaching at Yale and Columbia to go to UChicago because of the academic freedom has a book coming out Why We Fight: The Roots of War and the Paths to Peace that is research-driven rather than opinion or theory. I heard him interviewed by Tyler Cowan, who often has guests operating at depths beyond mine. Blattman made the observation that there is a U-shaped curve of politicians avoiding war, with those who have never been in the military and those who have been in combat both being more war-avoidant compared to those who were in the military but never so combat.
I had seen this as an opinion offered about American politicians, but did not know that it applies to other nations as well. I haven't read the book, I don't know his data set, but given his other comments I will provisionally take him at his word on this. It does make a sort of sense, though what level of cynicism you apply to the interpretation is likely as revealing about you as about the politicians. You could look through one end of the telescope and say that those who were in the military but did not see combat have a tendency to to be over-ready to be interventionist, as they "missed their chance." Or you could look through the glass the other way and say that those who saw combat are much more willing to say "Last resort."
I suspect there are nuances as well. John McCain was not eager to bring us to war, but once in, he became very hawkish in "We simply have to win. Losing would be a catastrophe." That's about where I lean, but it's not an area of any expertise for me. I am now convinced that because we can no longer win wars, we should probably be even more reluctant to enter them. It's our own damn fault. I would say that it didn't have to be this way, but given bureaucratic sclerosis over time, perhaps it did. When we gave up isolationism we chose a different path. We made choices to be a large bureaucratic military organisation, preferring those problems to the problem of catching up on the fly in an emergency, as we did in the 1940s. Maybe being unprepared and having Pearl Harbors or the sinking of merchant ships in 1917 is in the long run the better price to pay, ugly as it is.
There is also the theory that no one attacks a country that is economically on the rise, whatever its military. I doubt that is quite true, though I imagine there is something to it.
However, those "nuances" may apply only in the powerful nations, not in general. Of related interest, Blattman relates the tolerance for internal violence, present in most places but with very different expressions, to war attitudes.