Not necessarily. If the factories that build the robots are taken out, for example. Someone (even a robot) still has to build them.

What if the factories are located in foreign countries and the belligerents are only buying off-the-shelf products ?

Wars are always bad news and robot wars are very bad news. Many countries will fall into an endless war economy.

It turned out to be pretty hard to take out Germany’s factories in ww2.

Of course it did. War is hard, and lots of people die in it. The parent comment said always. I am saying that always is not true. I made no suggestions it was easy.