Image: UbisoftOfficial screenshot from Ubisoft.
All Is Fair in Politics is the story achievement for completing Memory Sequence 7 of Assassin's Creed Syndicate. It unlocks after Sequence 7, the political sequence in which the twins deal with Disraeli, Gladstone and a Templar in Parliament.
Play the main memories of Sequence 7 through to the end. Optional objectives do not matter for this one; only the 100% sync achievement, Mentor, asks for those.