The reason a Dutch mercenary captain was anywhere near Werl in 1586 began three years earlier with a wedding. In late 1582, the Archbishop-Elector of Cologne, Gebhard Truchsess von Waldburg, converted to Calvinism and declared religious parity for Protestants and Catholics in his electorate; in February 1583 he married Agnes von Mansfeld-Eisleben. The cathedral chapter responded by electing a competing archbishop, Ernst of Bavaria, and the Cologne War began. By 1585 the two sides were stalemated, and each looked outside the Empire for help. Ernst called on his brother and on Alexander Farnese, Duke of Parma, the formidable Spanish commander based in the Netherlands. Gebhard turned to the Dutch rebels and to the freelance Protestant commanders who served them. One of those commanders was Martin Schenck.

In March 1586, Schenck crossed the Rhine into the Duchy of Westphalia with five hundred infantry and five hundred cavalry, accompanied by Hermann Friedrich Cloedt, the commander of the fortress town of Neuss. Their stated mission was to secure two key fortifications, Recklinghausen and Werl, for Gebhard's cause. Their method was less strategic and more straightforward: they plundered everything they passed. Hamm, Soest, Unna, Vest Recklinghausen, and Waltrop all lost goods, livestock, and pieces of their churches to Schenck's troops, who stripped sanctuaries of icons, tapestries, and furnishings and, in Soest, abused the local clergyman. The Westphalian farmers and merchants who lost their salt, grain, and silver were not necessarily opposed to Protestantism. They were now firmly opposed to Gebhard.

Werl was rich for one main reason: salt. The town sat on the saline springs of the Hellweg, and salt was the medieval currency of preservation. On 1 March 1586, Schenck loaded a train of wagons with his soldiers and covered them with salt. The wagons rolled up to the town gates, and the guards saw what they expected to see - a valuable shipment of an essential commodity - and opened the gates. The salted soldiers came up out of the wagons, overpowered the watch, and took the town. But some of the Werl defenders escaped into the citadel, the small fortified core of the town, and there they held. Schenck stormed it several times. The citadel kept turning him away. Stuck with a town he had taken but a fortress he could not, Schenck did what mercenaries of the period did. He thoroughly sacked Werl, in part as a warning to its citizens not to help the citadel guards holding out at its center.

Word reached Count Claude de Berlaymont, also called Haultpenne, that a Protestant mercenary had seized a major Westphalian town. He brought four thousand men and heavy artillery to surround it. Schenck, with perhaps five hundred fighting men left inside the walls, was now besieged outside and harassed inside by the hundreds of defenders still holding the citadel. He tried once to break out and was thrown back, leaving fifty of his own men trapped outside the closed gates. Those fifty fled into the surrounding forest and went on raiding farmsteads, waiting for their commander to try again. Inside, Cloedt and Schenck loaded the wagons one more time, this time with the booty of the sacked town. They took thirty of the town's magistrates as hostages. Then they charged Berlaymont's lines. The breakout killed about five hundred of Berlaymont's troops and cost Schenck two hundred of his own. The mercenaries fought their way through, crossed the Rhine above Dortmund, and were gone.

Schenck personally did well out of the Westphalian campaign. He deposited his fortune and his wife in Venlo and reported to William of Orange in Delft. Robert Dudley, Earl of Leicester, acting as English Governor-General of the Dutch on Elizabeth I's authority, knighted him and gave him a gold chain valued at a thousand pieces. Gebhard's cause did very badly. The campaign secured no lasting fortresses, alienated the Westphalian population it had needed to win over, and demonstrated that any aid from the eastern Protestant princes would have to fight through a Spanish army to reach him. When Parma's army was finally committed in earnest, the military balance shifted permanently to the Catholic side. The loss of the Cologne archbishopric to Ernst of Bavaria consolidated Wittelsbach power in northwestern Germany, anchored a Jesuit Counter-Reformation stronghold on the lower Rhine, and gave Spain the river bridgeheads from which it would attack the Dutch Republic for the next sixty years. A trick with a wagonload of salt had ended exactly the wrong way.