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House Speaker John Boehner's resignation announcement was a historic event. Very few speakers have resigned mid-term, and indeed, we could only find five others in history: Jim Wright, who resigned in the face of scandal; John Nance Garner and Schuyler Colfax, who resigned just hours away from the end or beginning of their terms to assume the post of vice president; Andrew Stevenson, who was nominated as ambassador to the United Kingdom; and the singular Henry Clay. Clay actually served as speaker three times and resigned the post twice, once to negotiate the Treaty of Ghent that concluded the War of 1812 and a second time for a much more prosaic reason: He was broke.

Assuming no scandals come to light, this makes Boehner's resignation unique: He simply couldn't handle his unruly Republican caucus.

But Boehner was hardly the first to deal with a fractured or rambunctious party, so why did things go so awry for him? Because the GOP is not just split, it's split between two implacable factions: The dying realists, who are very conservative but still understand that politics is the art of compromise, and the surging nihilists, who are also very conservative but will accept nothing short of total victory and view any partial successes as utter betrayals.

Thus this war over the soul of the Republican Party is about something far deeper than ideology—it's about the very notion of coexistence with beliefs antithetical to your own. Some Republicans can live in that universe, but many more cannot. Boehner belonged to the realist caucus, a position the nihilists have made miserable to occupy. It's no wonder he quit.

And it's gotten far worse over time. Below, we analyze the evolution of the House across the decades, with a close look at three different congresses: One from the late 1960s, one from the mid-90s, and the one that began in 2013.