We do, in fact, have the 2nd amendment (right to bear arms) specifically so we can unseat any tyrant who tries to take our rights away.
Oh puhleeez. Seriously? You think the weapons that civilians have on hand can take on the best-funded military the world has ever seen? You know, the one that has more resources than the next five biggest militaries COMBINED? I don't think you've thought about this very seriously. Yes, I know that's the same thing "they" said about facing down the British back in 1775, but we're living in a different world. How many civilians have access to Abrams tanks and Apache helicopters? Cruise missiles? Not to mention: in what terrain has the US military been *training* for the last two hundred years? To paraphrase an awful movie, "When the day comes that we have to go to war against Utah, we're [the US military] really gonna kick ass".
The 2nd Amendment right puts guns in the hands of civilians. Thick-headed civilians who can't think very far ahead work like gang members who get their first chrome-plated .45: they immediately feel invincible, and that leads to the assumption that the guns are for the purpose of standing watch over the government. Don't kid yourself that any politician in the history of the US has ever thought (or will ever think) to themselves "well, I WANT to do X, but since the citizens have so many guns, maybe that wouldn't be a good idea."
If you want to know what the 2nd Amendment is really for, look no further than St. George Tucker, a lawyer, Revolutionary War militia officer, legal scholar, and a U.S. District Court judge (appointed in 1813), who wrote about the amendment: This may be considered as the true palladium of liberty... The right of self-defense is the first law of nature; in most governments it has been the study of rulers to confine this right within the narrowest limits possible. It is a right to defend yourself from the government, not an establishment of the power to overthrow the government. It's an important distinction.