Recent North Korea tests of a long range ballistic missile (that can reach parts of western North America), and a second test of their nuclear weapons, doesn't change the strategic situation with the United States. That's because of the American GBI (Ground Based Interceptor) system. The GBI is a 12.7 ton ballistic missile that delivers a 140 pound "kill vehicle" that will intercept a ballistic missile before it begins its descent into the atmosphere. The GBI kill vehicle attempts to destroy the incoming missile, while avoiding decoys. The U.S. is installing 26 GBIs in Alaska (16 are already active) and four in California. Another ten are being installed in Poland.
The GBI can receive target information from a variety of source, mainly a large X-band radar and space based sensors (that can detect ballistic missiles during their initial launch.) The U.S. plans to install 5-10 GBIs a year over the next few years. Each GBI costs over $100 million (up to several hundred million dollars, depending on how many are built and how you allocated development costs.) The GBI can intercept ballistic missiles launched from as far away as 5,000 kilometers.
North Korea is more concerned with threatening its neighbors (especially South Korea and Japan), which they can also do with shorter range missiles. If Japan believes North Korea is threatening it with nuclear weapons, Japan can become a nuclear power in a year or two. This has long been a possibility, and Japan has not been quick to deny it. South Korea could also develop nuclear weapons, and would do it nearly as quickly. Both countries are also increasing their anti-missile missile capabilities.