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The history behind the "Gadsden", or once the "1st Navy Jack" is actually very interesting.
The rattlesnake was a depiction of unity between states, as one rattle is silent while the entirety of them can be heard by everyone around. To go with such a lethal animal really showed their powerful desire for personal liberty, as a single bite from a rattlesnake is often fatal. That's all paraphrased and whatnot, but do research it if you're interested.