Considered during the period of WWII I would say because the empire covered a huge extension, mainly in the Pacific, consisting of islands and atolls very far from each other and from the metropolitan territory.
Since they control lots of island groups which makes thing hard to border up and defend on all sides... Unlike in Germany which they had 2 fronts and it was easier to have bunkers to defend certain location...
The statement "insure domestic tranquility" would remind the states to work together cooperatively because they are the governing entities that can help to prevent domestic conflict.
