Arnulf took the leading role in the deposition of Charles the Fat. With the support of the Frankish nobles, Arnulf called a Diet at Tribur and deposed Charles in November 887, under threat of military action. Arnulf, having distinguished himself in the war against the Slavs, was then elected king by the nobles of East Francia.

In 890 he was successfully battling Slavs in Pannonia. In early/mid-891, Vikings invaded Lotharingia and crushed an East Frankish army at Maastricht. In September 891, Arnulf repelled the Vikings and essentially ended their attacks on that front. The Annales Fuldenses report that there were so many dead Northmen that their bodies blocked the run of the river.

As early as 880 Arnulf had designs on Great Moravia and had the Frankish bishop Wiching of Nitra interfere with the missionary activities of the Eastern Orthodox priest Methodius, with the aim of preventing any potential for creating a unified Moravian state. Arnulf failed to conquer the whole of Great Moravia in wars of 892, 893, and 899. Yet Arnulf did achieve some successes, in particular in 895, when the Duchy of Bohemia broke away from Great Moravia and became his vassal state. In his attempts to conquer Moravia, in 899 Arnulf reached out to Magyars who had settled in the Carpathian Basin, and with their help he imposed a measure of control over Moravia.