Satire is a literary device used to expose the shortcomings of individuals, governments, and societies. Authors may use satire in the form of humor, insults, hyperbole, understatement, and ridicule to reinforce particular points. Swift certainly uses satire in Gulliver's Travels to give vent to his frustrations about volatile human nature, inept governments, and biased scientists. For this question, I will discuss Swift's satire against pointless religious or politically-based conflicts.
In the book, Swift satirizes the political enmity between France and England through his recounting of Gulliver's experiences in Lilliput. There, Gulliver finds that he is the only giant in a land of tiny inhabitants. He is captured by his curious hosts but manages to earn his release after his petitions for liberty are accepted by the Lilliputian government. Part of the terms of his release involves his responsibility of fighting for Lilliput should it be attacked by Blefuscu, Lilliput's sworn enemy.
In this first part of Gulliver's journey, Swift satirizes the long enmity between England and France on the global stage, Tories and Whigs in the English parliament, and Protestants and Catholics within England's social structure. He criticizes the absurd and pointless antagonism between these parties by offering up ridiculous rationales for them. For example, the enmity between Big-Endians and Little-Endians is based on arbitrary differences about the correct way to crack hard-boiled eggs. Because his son cut his hand while breaking his eggs on the big end, the previous emperor of Lilliput stipulated that everyone must crack their eggs on the small end. Many Lilliputians disagreed and rebelled; at least 11,000 rebels were executed, and many sought refuge in Blefuscu.
Swift uses the ridiculous arguments about big ends and little ends to satirize the equally absurd enmity between the Protestants and Catholics of his time. Catholic monarchs like Queen Mary 1 ("Bloody Mary") executed Protestants during her reign, and the Protestant Queen Elizabeth, although initially tolerant of Catholics, turned against those very subjects when a faction of devout Catholics challenged her right to the English throne. It seemed that the main point of contention between Protestants and Catholics arose from differences in worship.
The Catholics differed from the Protestants in their beliefs about how communion should be celebrated, whether priests should be intermediaries between God and man, whether church services should be held in Latin or the language of the people, or whether priests should remain celibate or marry. Swift felt that it was absurd for both sides to war against the other based on such inconsequential differences. In the story, he draws attention to the absurdity of religious wars by offering up a spurious theological reason for the schism between Big-Endians and Little-Endians. At the heart of the conflict is a disagreement about the right way to interpret certain religious principles.
During the course of these troubles, the emperors of Blefuscu did frequently expostulate by their ambassadors, accusing us of making a schism in religion, by offending against a fundamental doctrine of our great prophet Lustrog, in the fifty-fourth chapter of the Blundecral (which is their Alcoran). This, however, is thought to be a mere strain upon the text; for the words are these: ‘that all true believers break their eggs at the convenient end.’
Likewise, Swift satirizes the enmity between Tories and Whigs by highlighting the ridiculous rationale behind the Tramecksan and Slamecksan hostilities. So, the above examples represent some of Swift's many satires throughout Gulliver's Travels.