When he became pope on 14 December 872 (after the death of Adrian II), John commenced to push back on the presence of Muslims in and around Italy. He considered the Muslim successes the result of bad Christians. He requested aid from Charles the Bald and others, but aid was not forthcoming, not even from local Christian leaders.
The incoming Muslims had, in some cases, settled in and made arrangements with local Christians to live peaceably near each other. He decided to declare a ban on forming alliances, but no one was buying it. This was seen simply as a ploy to increase the reach and power of the Papal States.
In 876 he himself traveled to the cities of Salerno, Capua, Naples, Gaeta, and Amalfi to form an alliance against the Saracens. He reinforced the walls of the city. The 846 raid had plundered the Basilica of Saint Paul Outside the Walls (which was its name, not just a description of its location). He fortified the basilica, the connected monastery, and even the nearby houses. He also formed a papal fleet.
John's predecessor, Adrian, had condemned Photios I, the patriarch of Constantinople. John reinstated him, hoping to appease the Byzantines and gain their support.
Unfortunately for John, his attempts to strengthen Rome drained the papal treasury. This is assumed to be the reason that, almost exactly one decade after the start of his pontificate, he was poisoned and then clubbed to death by his own clergy, on 16 December 882.
"Taking out the boss" was not the usual way to end the pontificate of a pope, but it was effectively employed more than once in the history of the Liber Pontificalis, especially in the 10th century starting with John. There were another half-dozen "eliminations" that are certain, and then maybe a further half-dozen papal deaths that were, let's say, a little bit suspicious.
But as tempting a topic that is, I've realized that Charles the Bald keeps getting mentioned, going back 12 years, and hasn't really been given his own entry, despite being the descendant and ancestor of those who have been highlighted. Time to put Charles in the spotlight and find out if he really was bald, or if that was a reference to some other characteristic. See you tomorrow.