Walter Leslie (field marshal)

In 1630, he transferred to the army of the (Catholic) Holy Roman Emperor and fought alongside his countryman John Gordon that summer in War of the Mantuan Succession in Northern Italy.

During that time, Leslie, Gordon and Irish Colonel Walter Butler remained in the service of Wallenstein but began to correspond with the Italian generals.

[3] Leslie wrote to one, calling him the "protector of all foreign cavaleirs," pledged his allegiance to the Ferdinand II and requested a promotion based on his service to the emperor.

[4] As historian David Worthington points out, the letter was timed to arrive on 12 January, the day before the Pilsen Agreement was signed by 47 of Wallenstein's officers to remain loyal to him.

Days later, the Italian generals and 18 other senior officers signed the Pilsen Reverse, a document which expressed their loss of faith in Wallenstein's loyalty and ability.

That night, Leslie dined with Butler and Gordon and they finalized their plans to assassinate Wallenstein and his senior officers.

[9] The next morning, Leslie, Butler and Gordon all took an oath with one of the loyalist officers, Ilow, to remain loyal to Wallenstein.

Adam Trčka, Vilem Kinsky, Christian von Ilow and a lower ranking officer, Niemann all attended, but Wallenstein declined due to illness.

Again, reports of events differ slightly in the small details but what is agreed is that at the end of dinner, Leslie, Butler and Gordon all stood up, drew their swords and yelled something along the lines of "Long live Ferdinand and the House of Austria!"

Leslie's career as a military officer, politician, diplomat, and imperial noble skyrocketed as a result of assassinating Wallenstein.

His military rank was increased to a lieutenant field marshal and head of the bodyguard for the King of Hungary, and he was given command of two regiments.

Historian David Worthington claims that of all the assassins of Wallenstein, Leslie was rewarded the most and ultimately his career grew the most out of that event.

In 1637, Leslie wrote to the dying Ferdinand II, making his argument that he had Habsburg blood dating back over half of a century.

Leslie served as a Stuart-Habsburg intermediary at the Ratisbon Electoral meeting (1636–43), Imperial envoy to the Spanish Netherlands (1639), diplomatic trips to the Caroline Court in Spain, delegate in Regensburg (1640), and ambassador to Naples in 1645.

[15] In 1645, Leslie, then Imperial Ambassador to Naples, roamed the Italian Peninsula in an attempt to garner funds to fight the war.

[16] Leslie was made a Field Marshal and the Governor of the Croatian-Slavonian Military Frontier in 1650, an appointment that drew his attention to the Southern reaches of the Holy Roman Empire.