A polemic is an often inflammatory text directed against a group of people, or against an idea. In dialogs about religion, an essay written against a particular faith is a polemic, and the reply defending the faith is an apologetic.
Polemic may also be political. The inflammatory deriding and appeals to emotion characteristic of partisan blogs, for example, may properly be termed polemic or invective.
Texts and quotations edited to remove single words, or particular details or elements, to prejudicially alter their actual meaning are made into polemic by deliberate omission.
A famous historical example is the Ems Telegram of 1870.