According to a confidential assessment by the Pentagon's Defense Intelligence Agency, North Korea will be able to field a reliable, nuclear-capable intercontinental ballistic missile as early as next year. (The Washington Post)

In this occasional series, we will bring you up to speed on the biggest national security stories of the week.

North Korea showed the world Tuesday what experts have known for a while: The regime is intent on building rockets capable of reaching the U.S. mainland.

The reclusive communist regime fired an intercontinental ballistic missile, or an ICBM. The missile traveled more than 1,700 vertical miles before landing in the Sea of Japan. On a flatter trajectory, experts say, this type of rocket would be capable of reaching as far as Alaska.

Here’s what you need to know about North Korea’s nuclear program and how the United States might respond:

What did North Korea actually accomplish this week?

The regime has previously modified outdated Soviet technology to build almost all of its ballistic missiles. For years, it has sought to develop an ICBM capable of putting U.S. cities within striking distance. As it stands now, the rocket that Pyongyang tested on July Fourth cannot reach the Lower 48 states.

The missile test was a success because it flew higher and remained longer in the air.

“It’s not a copy of a crappy Soviet engine, and it’s not a pair of Soviet engines kludged together — it’s the real thing,” said Jeffrey Lewis, director of the East Asia program at the James Martin Center for Nonproliferation Studies.

The true test of the missile will be when it’s in operational control, meaning when it’s ready to be used as a weapon. To do that, North Korea still needs to figure out how to build a nuclear warhead small enough to fit onto one of its longer-range missiles.

How did the Trump administration respond?

U.S. and South Korean forces mobilized early Wednesday morning to conduct joint military exercises that included firing missiles of their own. U.S. Pacific Command said the launches were a response to “North Korea’s destabilizing and unlawful actions.”

Dana White, a Pentagon spokeswoman, said that the commitment to U.S. allies is “ironclad” and that the United States is prepared to use “the full range of capabilities at our disposal against the growing threat from North Korea.”

Trump was quick to respond to North Korea’s launch on Twitter: “North Korea has just launched another missile,” Trump said. “Does this guy have anything better to do with his life?”

Kim Jong Un also took a turn to mock Trump. North Korea state media reported that Kim urged his scientists to send more Independence Day gifts to the Americans.

What did Russian and Chinese officials say?

The administration’s retaliatory plans are at odds with Russia and China — and it needs both countries on its side to have any effect on Pyongyang. Beijing and Moscow both oppose additional economic sanctions or any military action.

North Korea’s nuclear threat will no doubt be on the minds of both Trump and Chinese President Xi Jingping when they meet on the sidelines of the G-20 economic summit in Hamburg this week.

What are the options on the table for the U.S. and South Korea?

Before he flew to Germany, Trump warned there would be “some pretty severe” consequences to North Korea’s actions.

“Something will have to be done about it,” Trump said, without giving more details.

Trump had hoped Beijing would put more pressure on the regime and has even praised Xi for his approach to North Korea. Yet China has called for a measured approach and has been slow to act, irritating Trump enough to tweet that if Beijing doesn’t step in, the United States will go it alone. So far, Trump has kept to the same policy as Obama: isolate Pyongyang through diplomatic and economic means.

Any military action against North Korea would almost immediately cause North Korea to react. A massive amount of artillery is pointed toward Seoul, home to half of South Korea’s population. If North Korea were to fire against the South, experts say thousands of people would be killed within the first three hours.

This is one of the main reasons that previous administrations have used restraint in acting against Pyongyang.

Is the U.S. military close to acting?

In short, no. Defense Secretary Jim Mattis said the launch doesn’t mean the United States is closer to war and that the focus will remain on pressuring North Korea through economic sanctions and diplomatic measures. Despite the launch, the Pentagon has stressed that North Korea’s threat is “nascent” and that the country’s advancement in missile technology is still limited.

A previous version of this story incorrectly stated a majority of Seoul residents would be killed if North Korea struck  South Korea. Thousands would be killed.