High level- this was the product of the German military being led by the aristocratic junker class, who had class interests in conservativism due to their aristocratic and land owning nature, and professional and class interests in militarism due to their control of the military offices and bureaucracy.

In the German context, the junker landed class had a close relationship with the military, as many from that class would serve as officers. In Germany in the decades around 1900, there was a major cleavage in German politics between the junker class and the urban workers. Land was scarce relative to labor and capital, and the urban workers would benefit from greater economic opening whereas the agrarian junkers would prefer a motor closed economy where they could charge more for grain/food. Other policies that we would associate with liberal/urban interests would diminish the power of the junkers such as reforms to civil and military service, greater capital mobility, etc, setting up a rural-urban conflict in German politics, with the rural led by the conservative landed junker class.

In the lead up to WWI, the junkers saw their social position increasingly under threat, and some like Arno Mayer have argued that the rise in bellicosity and militarism in the German ruling class was a defensive measure against eroding social position at home - the more that German society was driven by/engaging internationally via a mercantile position, the more that the urban workers and entrepreneurs would benefit. By contrast, more international tension would push for a more closed economy that benefits rural and agrarian interests, and in times of potential military strife, the junker class would be in the proverbial drivers seat as the main class holding the military offices and posts.

After WW1, you would expect the junker class and their position to be discredited. However, 3 elements interferes with that: first, there was an influential narrative of the end of WW1 that posited that the German army would have won, if not for being betrayed by a weak civilian leadership who capitulated too soon - which reinforces the tie between conservatives and the military. Without an actual conquest of Germany taking place to cement their loss of the war, common people were susceptible to this narrative. Two, there was the massive decline in international trade in the inter war period, which benefited the old landed class over the new urban classes (see Rogowski, Commerce and Coalitions). Third, the failure of the Weimar gov and the rise of political unrest in Germany to the point of violence in the streets reinforces the ties between conservative politics and public order, and there was a phenomenon where WW1 veterans stepped in to a vigilante police role.

Regarding other countries, the dynamics are different. In Portugal during the Salazar-caetano regime, the military became a hotbed of left wing thinking and eventually helped overthrow the fascist gov. In Turkey in the 20th c, the military long proved a bastion of liberalism; Latin America also has examples of this pattern. On the role of the military as a force of progress va reaction, the classic work is by Sam Huntington .