Note that the HRE is not the same thing as the real Roman Empire. For half a millennium there were two political structures in Europe which called themselves "Roman Empire"; one was the real Roman Empire, and was a real state; the other was the bizarre German construction, and it was not a state.
That's why the "Holy" part in the name of the Holy Roman Empire is never left out, and why we generally call the postclassical Roman Empire the "Greek" or "Byzantine" Empire. Less confusion this way.
This is worth repeating. The HRE was not a state; if was a very loose confederation of many small or medium-sized states. Even the present day European Union is very much more of a state than the HRE: at least the EU does collect a small amount of tax from its member states, does forbid commercial barriers between the member states, and does insist on harmonizing the laws of the member states.
The states which made up the HRE paid no tax to the emperor. In fact, most emperors had very little power. (How much power the emperor had depended largely on how rich he was and how skilled he was at politicking.)
The states which made up the HRE were not compelled to place their armies under the command of the emperor, unless that course of action was agreed by the Diet (Reichstag in German, translated into English as Diet when speaking of the HRE and Parliament when speaking of the German Empire or the post-WW1 German Republic), which hardly neverever happened.
As a consequence, the HRE never ever waged war upon anybody as a coherent whole.
Eventually, the well-functioning part of the HRE split off and became the Austrian Empire. The remainder remained just a collection of small states, which were eventually assembled by Prussia into the short-lived and ill-fated German Empire.