I would mostly have to disagree. When it comes to ethnicity, we must also look at cultures within these groups to answer why certain kinds of people would choose to fight in the first place. much of this has connections to their religion, governments, where they live, their connections to outsiders of other cultures as well.
Ancient Germans, Celtics, Romans,Greeks, Arabs, Japanese, Assyrians and many others all over the globe would be seen as warrior nations because war is a major part of their religion. War gods in their polytheistic religions.
Next we must look at governments and their motivation to expand their realm. Roman Empire, Greece, Persian Empire, the Huns, Mongols; if they could win and expand, they would do it. All about influence and power. Such has continued through to the end of the Cold War.
When Islam began to spread, so did the hundreds of Jihads declared by the followers to non-Muslim areas. This is an example of how religion can make a prior non warrior nation of people into a culture that ready for a fight.
After the establishment of the UN there has not been a massive war with great losses. With the weapon tech of today, it is all about who wants to start the fight as body-build means little now. anyone can take on anyone if both are up to fight. After WWII, no one wants to have another killer like that. However with the great amount of unrest in both large and small nations, when will the spark hit the keg?