The main goal to demiliterise Germany was to ensure that she could not hold aggression and threaten the other nations across the border.
Rhineland of Germany was demilitarised as there are other nations nearby, and it is the shortest route to countries like France. It is therefore demilitarized with all troopa removed. This eliminated the potential of Germany launching a surprise attack in aggression, thus fufilling the goal of fourteen points to keep peace across nations.
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