Port security past

In 2003, North Korea was the one to fear. Via GlobalSecurity.org:

Democratic Senators Fritz Hollings (South Carolina), Charles Schumer (New York) and Patty Murray (Washington) issued a press release March 6 urging the Bush administration to immediately boost security at U.S. border crossings and ports because of the threat posed by North Korean nuclear processing.
The release states that "as reports of North Korea's new ability to produce, and possibly sell, nuclear materials to terrorists have come to light," more resources must be provided to prevent these materials from entering the United States for use in terrorist activities.
"Failing to secure our ports from attack could result in a catastrophic attack on our economy and, ultimately, on the health of our nation," Hollings, the top Democrat on the Senate Commerce, Science and Transportation Committee, said in the release.
The release states the Bush administration is "hampering congressional efforts to protect our seaports from attack" by refusing to adequately fund measures such as the Port and Maritime Security Act, which was signed into law December 2002.

Securing the US port may be irrelevant to such a scenario. According to this Customs & Border Protection Today article from 2003, terrorists could simply detonate a nuclear device while moored port, entirely bypassing homeland defenses.

...terrorists planning to trigger a nuclear device hidden in a sea container don't have to guarantee the container makes it through a U.S. port to be certain of its destructive effects; if one of these devices explodes, it can do just as much damage from the deck of a ship temporarily moored outside one of our ports or moving slowing along the U.S. coast. A "dirty bomb" doesn't have to make it all the way into a department store in LA to deliver its murderous payload. Five miles offshore, or anchored for a few hours outside the port of San Diego-it's still a perfect opportunity for terrorists to detonate a radioactive device and establish a "kill zone" extending hundreds of miles inland.

Although U.S. lawmakers seem fixated on homeland defenses, it's obvious that U.S. port security must begin overseas. Waiting until it gets near shore is too late.