These men, uniting in bands for self-protection and taking their lives in their hands, were able with good luck to take themselves but little else across this central region and the western barrier.

All ordinary communication, all mail and all freight, must go by sea.

The United States was actually divided into two very unequal parts, and California and Oregon were geographically far distant colonies.
The ocean highroad belonged to the United States in common with all nations, but it took American ships to the opposite ends of the earth.
No regular shuttle of traffic sufficient to weave the nation together could be expected to pass Cape Horn at every throw.

The natural route lay obviously through the Caribbean, across some one of the isthmuses, and up the Pacific coast.

Here however, the United States would have to use territory belonging to other nations, and to obtain the right of transit and security agreement was necessary.