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In their after-dinner conversation, they discussed politics.
Literal
Dinner-after-of conversation [at-で] they [topic-は] politics [about-について] discuss-[past-った].
Several vocabulary points. 談話 ('conversation, discourse, talk') is a formal word for talk or discussion, rarer in casual speech but common in writing. 政治について ('about politics') uses について, one of the standard ways to mark 'about X' for topics of discussion. 話し合う ('to discuss, talk with each other') uses the reciprocal ~合う to mark mutual action — 'to talk together.'