‘l. mperialist’ in the 16th Century was not yet a political expletive, but referred to the forces of the only European Emperor, who presided more or less shakily over a conglomeration of semi-independent states stretching from the Rhine to Poland, from the Danish border to the edge of the Turkish Empire.
The selected Emperor was largely dependent on the troops and cash he could beg from the Imperial Diet or borrow from the Princes. There was no standing army till the end of the 30 Years’ War, and it was then a modest force of nine regiments of foot and ten of horse, supported by the hereditary Hapsburg lands of Austria and Hungary. Hence the fact that the Lansknechts and Reiters spent most of their time and gained their chief fame, in foreign service.
Chief external enemies for the Imperialists were the Turks and the French, but they were also occupied against internal Protestant opposition, both in the 16th Century and in the vast and horrifying conflict of the 30 Years’ War (1618-48) which I have taken as the close of our period.
The mass of a 16th Century Imperial Army would be of Lansknechts with pikes and firearms, but under Charles V (1519-58) Spanish troops would also be available, and they were in fact lent as late as the 30 Years’ War, while Imperialist infantry normally followed
Spanish tactics, with very large pike-squares surrounded with shot.