Two Viking armies, the established force in…

The Romans had taken the offensive in the age-long struggle between Christian and Muslim on the eastern borders of Asia Minor from the mid-ninth century onward.

Basil, taking advantage of turmoil within the Muslim caliphate, has continued the attacks made during Michael III's reign against the Arabs and their dualistic Christian allies, the Paulicians.

Basil initiates a war of expansion in 871 by dispatching an army to the upper Euphrates River at Samosata (Samsat).

Raids across the eastern frontier into the Euphrates region continue, though …

…Basil does not manage to take the key city of Melitene.

The caliphal armies, now entrusted to al-Muwaffaq, a brother of the new caliph, al-Mu'tamid, still cannot cope with the rebels.

The Zanj sack Basra in September 871.

The Arab governors of Multan and …

…Sindh break away from the caliphate in 871 to establish independent kingdoms apparently characterized by partial religious tolerance.

Emperor Louis II has spent most of his career trying to expel the Saracens from southern Italy, although the Lombard princes have hindered him in this endeavor.

Basil, whose plans for Italy have involves him in negotiations with Louis the German, king of the East Franks, launches an expedition against the Saracens in Sicily and southern Italy, capturing Bari in 871, his sea forces aided by the land army of Louis II.

Some jealousy between Louis and Basil follows the victory at Bari, and in reply to an insult from the eastern emperor Louis attempts to justify his right to the title "emperor of the Romans."

Louis has withdrawn into Benevento to prepare for a further campaign when he is treacherously attacked in his palace, robbed and imprisoned by Adelchis, prince of Benevento, in August 871.

The landing of fresh bands of Saracens compels Adelchis to release his prisoner a month later, and Louis is forced to swear he will take no revenge for this injury, nor ever enter Benevento with an army.

A report that the emperor Louis II was dead leads to peace between father and sons and attempts by Louis the German to gain the imperial crown for Carloman.

These efforts are thwarted by Louis II, who is not in fact dead, and Louis' old adversary, Charles the Bald.

King Æthelred of Wessex, and his brother and successor, Alfred, the first English monarch to plan systematically for the defense of his realm against the Danish Vikings, war with them from 871, winning a victory at the Battle of Ashdown, in Berkshire (possibly the part now in Oxfordshire), at the beginning of the year.

Accounts of the battle are based to a large extent on Asser's "Life of Alfred", however there is some dispute about whether this is an authentic account.

Alfred, now king of Wessex, attacks in late 871 near Wilton after a series of inconclusive skirmishes between the Danish Vikings and the West Saxons.

The Danes feign a retreat in a successful ploy to draw an attack, and are victorious.