If $|G|=p^{n}$
Then
Why is it that every subgroup of order $p^{n-1}$ is normal?
If $|G|=p^{n}$
Then
Why is it that every subgroup of order $p^{n-1}$ is normal?
A subgroup of index $p$ where $p$ is the smallest prime dividing the order of the group is a normal subgroup.