Divide et impera, a Latin phrase meaning divide and rule.
The origins of the phrase are unclear. Some assign the authorship to Niccolo Machiavelli (1469-1527), others date it back to the days of the Roman Empire. According to the Random House Dictionary of Popular Proverbs and Sayings, it "has been in common use since M. Hurault's Discourse upon the Present state of France (1588)".
It appears to have been the motto of Philip of Macedon (lived 382-336 BCE, ruled 359-336 BCE) who predates the Roman Empire let alone Niccolo Machiavelli and Hurault. He also predates the popularity of Latin, so here we see the Latin translation of his motto rather than his original motto.
Later, it also became the motto of Louis XI (lived 1423-1483, was King of France 1461–1483). It also was the traditional motto of Austria (that is rather ironic considering that the huge Austrian Empire was completely divided up in 1918 and left Austria as a fairly small country). The phrase was also used by Polybius, Bossuet, and Montesquieu.
No matter who was the first to use this phrase, it has been applied by political strategists since times immemorial.