Chicago's second album, simply titled Chicago but at times retconned to Chicago II, is what really broke the band.  The singles from Chicago Transit Authority didn't exactly light up the charts in 1969, but a year later they were re-released and shared the spotlight with the ones from Chicago.  The band went on a massive tour and then right back into the studio for Chicago III, having to come up with all new material.  In contrast to the hit machine that was Chicago, Chicago III saw the band taking the jazz part of their much more seriously, as well as adding in some funk and R&B.  It was decidedly non-commercial, and would be their last double album for quite awhile.