The Strongest Poison ever known Came from Caesar's Laurel Crown.
The American diplomat Henry Kissinger once said, "Power is the ultimate aphrodisiac." Maybe so—but, according to Blake, it's also poison (poison, we tell ya!).
The desire for absolute power ultimately ruins and destroys the people who get it, says Blake—either they get assassinated like Caesar or they become monstrous tyrants like Stalin and Hitler. Either way, it's terrible.
The Romans used the "laurel crown" to crown victorious conquerors (like Caesar), by the way. But it's also used in a peaceful way, to crown people who've been victorious in other pursuits—like the arts. (Blake thinks that imagination, art, and spirituality are a pretty strong antidote to the poison of power.)
The word "laureate" (as in "poet laureate" or "Nobel laureate") is a reference to these laurels. Bay laurels (the kind typically used) aren't really poisonous either—people frequently use them to flavor pasta sauce. (Mmm… pasta.)
Also, "known" and "crown" form another imperfect rhyme to our modern ears.
Nought can deform the Human Race Like the Armour's iron brace.
This is another anti-militarism couplet, like the couplet of lines 77-78 (and sort of like the one just before at 97-98, since Caesar was the leader of a military empire). But it also has secret, inner depths.
The basic meaning is pretty clear: when the human race gets dressed up to go on a killing spree, it looks deformed. We're not at our best when we're in destruction mode.
According to Blake, what makes us truly human isn't our ability to conquer and destroy, but to create and love (this is the subject of the next couplet, after this one). So, the "Armour's iron brace" deforms in that sense.
But the armor is also a good metaphor for what human selfishness and greed do—they seal you off from reality and wrap you in a personal cocoon. It prevents you from being open to whatever reality—especially spiritual reality—might be there.