If oceans are barriers what do they bar. Not the ships that the original settlers sailed here on.
Oceans bar travel the way that red lights can stop cars.
It means if someone wants to make trouble with the US it will be an attack on the mainland. At that point our politically divided nation will agree we must fight. When we go overseas to start a war to prevent a war, we end up limiting ourselves due to geo political realities, or we must nation build and subject our men to ROE and irregular warfare. This formula has been played from Korea, Vietnam, and now Iraq/Afghanistan. US domestic characteristics limits us to defined short wars which our leaders are unable to keep when going to war overseas. Result is eventual defeat as the war drags on, enemy peppers us to death with IED and the cost to a nation already deep in debt. In the meantime the party out of power will demonize the war with the hope of taking power from the party in power. It all ends badly for the poor troops who have to put up with all this. Many of our enemies have to climb thru many neighboring nations before they can physically launch a seaborne invasion of the US/Western Hemisphere. Our country needs another Pres Eisenhower who will usher a period of quiet as our nation pay down our debt and rebuild her strength and forces. The US did not become a great power via a conqueror like Napolean, rather it was by letting the powers of the day kill each other till both are at the brink of collapse that we plunged into the fight fresh and untouched. The US should not be killing herself to maintain the peace in the world. Let the nations fight amongst themselves again. Give war a chance. When the US is no longer dominant in Europe and Far East Asia, China and Russia will eye each other with suspicion. I predict they will end up fighting with each other over the EuroAsia regions. In a multi polar world, the first two major powers fight will lose. Be the monkey on the rock watching two lions fight will be the winner.