Definition: The Saxons were an early Germanic tribe that would play a significant role in both post-Roman Britain and early medieval Europe.
From the first few centuries B.C. up until about 800 C.E., the Saxons occupied parts of northern Europe, with many of them settling along the Baltic coast. When the Roman Empire went into its long decline in the third and fourth centuries C.E., Saxon pirates took advantage of the reduced power of the Roman military and navy, and made frequent raids along the coasts of the Baltic and the North Sea.
In the fifth century C.E., Saxons began to expand fairly rapidly throughout present-day Germany and into present-day France and Britain. Saxon migrants were numerous and dynamic in England, establishing -- along with several other Germanic tribes -- settlements and power bases in territory that until recently (c. 410 C.E.) had been under Roman control. Saxons and other Germans displaced many Celtic and Romano-British peoples, who moved westward into Wales or crossed the sea back to France, settling in Brittany. Among the other migrating Germanic peoples were Jutes, Frisians, and Angles; it is the combination of Angle and Saxon that gives us the term Anglo-Saxon for the culture that developed, over the course of a few centuries, in Post-Roman Britain.
Not all Saxons left Europe for Britain. Thriving, dynamic Saxon tribes remained in Europe, in Germany in particular, some of them settling in the region that is today known as Saxony. Their steady expansion ultimately brought them into conflict with the Franks, and once Charlemagne became king of the Franks, friction turned to out-and-out war. The Saxons were among the last peoples of Europe to retain their pagan gods, and Charlemagne became determined to convert the Saxons to Christianity by any means necessary.
Charlemagne's war with the Saxons lasted 33 years, and in all, he engaged them in battle 18 times. The Frankish king was particularly brutal in these battles, and ultimately, his ordered execution of 4500 prisoners in one day broke the spirit of resistance the Saxons had displayed for decades. The Saxon people were absorbed into the Carolingian empire, and, in Europe, naught but the duchy of Saxony remained of the Saxons.
Examples: Charlemagne's war with the Saxons was long, bloody, and brutal.
The Saxon invasion of England was less an organized conquest than a long sequence of migrations of families, clans, and warrior-groups.