I find that the army fought by the Prussians at Rossbach is consistently referred to as a "Franco-Imperial" army-- German sources sometimes refer to it as a "Reichsexecutionsarmee" (Imperial execution army).
At Leuthen, a few weeks later, the Prussians fight an army that is consistently referred to as "Austrian", but there were also regular Austrian army units in it, such as the Bretlach Cuirassiers who also fought a year prior in the Battle of Prague, in the Austrian army.
So, my questions: What makes the Rossbach army "Imperial"? I'm assuming it was somehow raised or sanctioned by the Imperial Diet? Were there any restrictions on the Habsburgs in their function as Austrian monarchs how these Imperial units could be used? The army was commanded by an Imperial Prince at Rossbach-- was that a requirement or could it have been led by an Austrian officer?
I suppose I'm just looking for an explanation in how these Imperial armies functioned in that difficult legal situation where the Habsburg domain and the Empire were almost--but not entirely!-- the same.
Thanks in advance!
The Holy Roman Empire is weird. Let me see if I remember correctly...
On the one hand we have the Reichsarmee (Army of the HRE) and then we have the Kaiserliche Armee (Imperial Army).
The Imperial Army was simply the army raised by and for the use of the Emperor, so at that late point in time effectively the Austrian Army - although from 1742-1745 during the War of the Austrian Succession the Wittelsbacher Charles Albrecht of Bavaria held the title of Emperor as Charles VII., making his Bavarian Army de facto the Imperial Army while the Habsburg troops were subject to Maria Theresia in her role as Queen of Hungary since there female inheritance of the throne never had been an issue so it didn't get disputed by her rivals.
It's worth noting that while that army is often called Austrian as a shortening it would actually be recruited from all parts of the Empire (except prince-electorates as one of their special perks) and have some other niceties like universal passage.
And then there's the Reichsarmee. This was still the Emperor's army in the sense that he was its supreme commander but he had no free reign over them otherwise. This army was raised by the Imperial Diet for specific purposes and came out of ten different "Imperial Circles" of the Empire, who each had to provide and keep standing a certain amount of troops in case there was need for them. Of course, in practice this often meant they'd hand some peasant a few sticks and tell them to go die for the Reichsarmee while they kept their best troops for personal use. Not to mention that recruiting from so many different military structures that never came together until immediate need led to some weird issues that made battle deployment difficult because of variable marching speeds and such.
Reasons for the Diet to raise the Reichsarmee were normally either external wars, like in the War of the Spanish Succession, or an internal Reichsexekution, which basically meant sending an army to enforce an Imperial decree or a decision of the Reichskammergericht (supreme court of the HRE) in a member state. This would include the Reichsexekution against Prussia in the Seven Years' War for their illegal occupation of Saxony.
For your question of Roßbach vs Leuthen the answer would be that for Roßbach the required Reichsarmee had been raised (with considerable delay) and was deployed... under French command, no less. While at Leuthen it was specifically only Austrian (Imperial) troops under Prince Charles Alexander with only minor support by some Bavarians and Wurttembergians.
Sources: Mostly from memory. I think I got the main gist of that from Christopher Clark's Iron Kingdom: The Rise and Downfall of Prussia, 1600-1947