This is a jab at John Churchill, Duke of Marlborough (1650 - 1722), who managed to enrich himself during the course of his military career and who was charged by his enemies with avarice and corruption. Swift was one of his most biting critics, and in 1710 he wrote a savage indictment of Marlborough in which he listed all his earnings and had them come to 540,000 pounds, equivalent to tens of millions of dollars in present-day money. Indeed, Swift implied that Marlborough was prolonging the war in order to further enrich himself.
The Duke of Marlborough died in 1722, four years before Gulliver's Travels was published. He left behind one of the finest country houses in Britain, Blenheim Palace, funded in part by "a grateful nation" and partly by the Duke.