The United States should expand the US Coast Guard to secure 100% of our borders, not just those abutting water. The US Coast & Border Guard would give us a unified defense against terrorist and smugglers, and would force the much needed reform of immigration (a separate but related issue).
It would be necessary for Congress to work within the spirit of the Posse Comitatus Act to ensure that a federal Army was not exceeding this mandate, which could simply be done by requiring all arrested individuals to be turned over to civilian authority with a reasonable time frame.
This expansion would help the economies of border states, and increase our overall security.