Carefully read "On War" pages 75-89 (attached) . This is sort of an "executive summary" of the contents as a whole.

On pages 75-78 Clausewitz introduces the concept of "war in theory." He also uses the terms like "ideal" or "in the abstract" but they all mean the same thing.

Clausewitz sees that war's natural tendency would be to move to extremes in terms of violence, efforts, and purpose.

This is nicely summed up in the third paragraph on page 78.

What factors transform "war in theory" into what Clausewitz calls "war in reality?"

Just a note. Don't misunderstand, Clausewitz does not advocate for war to be fought as it might be fought in "war in theory."

He sets up "war in theory" as an "ideal type" ( likeIssac Newton and frictionless physics) so that we may examine war as it is actually conducted, and why war "in reality" unfolds as it does.

Many readers mistake the of interpreting Clausewitz term "ideal type" as how a war should be conducted. He doesn't.