In the mid 19th century many people in America had a very poor diet. Open land had made meat cheap. Sugar was an inexpensive way to make starches tasty. Basically the same problems that we complain about today. Too much meat, starch and sugar not enough fiber. I'm not saying it was the sole cause of the US Civil War but the number of constipated people walking around couldn't have helped.

In 1863 a man named Jackson invented Granula which used a completely different recipe but somehow managed to be very similar to the Granola we have today, tasteless, crunchy, mostly inedible but high in fiber. To make Granula edible it had to be soaked overnight. For many years Milk had been used to sweeten hardened breads (think of French Toast as an example). Naturally milk was used to sweeten and soften Granula.

A major health craze swept through the United States in the second half of the 19th century. Cereal was a major part of this craze rolled oats, Granula, Grape Nuts, Shredded Wheat, Wheat flakes were all invented as 19th century health food. Of course the mantra of health food then is the same as it is today "the worse it tastes the better it is for you." Milk or juice with the cereal was a nice way to bend this rule without breaking it. Eventually Milk won the battle against juice merely because it was available year round while juice tended to be more seasonal.

To find out the rest you just need the magic code word KELLOGG Everything breakfast cereal related revolves around that name and a Sanitarium in Battle Creek. and cue dramatic music Dun Dun DUuuunnn. But seriously look it up there are some amazing stories to be found.