The Holy Roman Empire that Otto I controlled extended over the German plain north to the Baltic, eastward into parts of modern Poland, and southward through modern Switzerland, modern Austria, and northern Italy. From the first, the emperors had a difficult problem keeping control of two disparate regions, Germany and Italy, that were separated by the Alps.

The Holy Roman Empire was successful at first because it benefited the principal members, Germany and Italy. The Germans were not far removed from the barbarian condition. They had been conquered by Charlemagne only a century earlier. They benefited greatly from Italian culture, technology, and trade. The Italians welcomed the relative peace and stability the empire insured. Italy had been invaded time and again for the previous 500 years. The protection of the empire defended the papacy and allowed the city states of Italy to begin their growth.

The imperial armies were manned partially by tenants of church lands who owed service to the emperor. A second important contingent were the ministriales, a corps of serfs who received the best training and equipment as knights, but who were not free men. These armies were used to put down revolts or interference by local nobles and peasants, or to defend against raids by Vikings from the north and Magyars from the east.

Because Germany remained a collection of independent principalities in competition, German warriors became very skilled. They hired out as mercenaries and sent large contingents to the Crusades. The most renowned German soldiers were the Teutonic Knights, a religious order of warriors inspired by the Crusades. The Teutonic Knights spread Christianity into the Baltic region by conquest but were eventually halted by Alexander Nevsky at the battle on frozen Lake Peipus.