The American political system is being challenged in unprecedented ways, but that doesn't mean it hasn't been challenged before.The Mueller investigation into Donald Trump is just the latest of over a dozen special investigations ordered since the US Civil War.But there are two in the last 50 years which are seen as historical references for what’s happening right now - Watergate, and the Lewinsky Scandal.One led to a resignation. The other led to an impeachment.We spoke to Leon Neyfakh, a man who has delved deeply into both those investigations for his wildly successful podcast, Slow Burn, and learned a lot of lessons which should be remembered while talking about the Russia investigation.