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A timeline of North Korea’s five nuclear tests and how the U.S. has responded

Military officers visit the birthplace of North Korean founder Kim Il Sung in Mangyongdae, just outside Pyongyang, on Friday, a day before the 105th anniversary of his birth. (Damir Sagolj/Reuters)
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April 15 is the most important day on the North Korean calendar — it's the birthday of the founder of North Korea, officially known as the “Day of the Sun.”

This year, experts and analysts warn that North Korean leader Kim Jong Un might use the day as an excuse for a dramatic show of force. As my colleague Anna Fifield reported:

Expectations remain high that North Korea will conduct another nuclear or missile test, or carry out some other incendiary act, to mark the most important day on its calendar: the anniversary Saturday of the birthday of Kim Il Sung, the country’s founder and the current leader’s grandfather.

In recent days, the North Korean army has threatened to annihilate U.S. military bases in South Korea in response to what it called President Trump’s “maniacal military provocations.” In response, the Trump administration warned that all options are on the table. The United States has sent an aircraft carrier strike group to the Korean Peninsula region. Trump has tweeted that the United States will act against the country, with China or independently. It's gotten so contentious that China has publicly chastised both countries and asked them to chill.

As the world waits to see what — if anything — North Korea does, here's a look back at the country's five nuclear tests and how the United States responded:

October 2006: Analysts determine that North Korea has conducted its first nuclear test. The test produced an explosion of less than one kiloton, or the equivalent of about 1,000 tons of TNT. That is a small fraction of the size of the bombs dropped by the United States on Japan at World War II's end.

The United States considered this test a failure. Even so, American officials pushed for tough sanctions, calling for a block on all imports of military equipment to North Korea. Eventually, the United Nations passed a less strenuous measure, specifically targeted to prevent North Korea from acquiring equipment that would help it expand its nuclear program or military.

Kim Jong Un has tested nuclear weapons at an unprecedented rate. Here's how his regime is able to funnel billions of dollars into its nuclear program. (Video: Jason Aldag/The Washington Post, Photo: Linda Davidson/The Washington Post)

May 2009: North Korea conducts its second nuclear weapons test. The test was carried out underground. At the time, the U.S. Geological Survey recorded a magnitude 4.7 seismic disturbance. National Intelligence Director James R. Clapper Jr. later estimated that the test produced an explosion of two kilotons.

President Barack Obama called the test a “grave threat,” but military officials said this was a diplomatic, not a military, matter. The United Nations imposed tighter sanctions on North Korea — now almost all arms imports were banned. It also called for intensified weapons inspections.

February 2013: Kim Jong Un, then newly in power, conducts his first nuclear test as leader. The test was far larger than earlier experiments, with experts estimating that the bomb was between six and seven kilotons. The test coincided with South Korea's national elections and Obama's State of the Union address.

In response, the United States moved some missile defense equipment and nuclear-capable stealth bombers to South Korea. Secretary of State John F. Kerry warned that North Korea would lose in a military showdown with the United States. Kim “needs to understand, as I think he probably does, what the outcome of the conflict would be,” Kerry said. In the wake of the test, the United Nations once again moved to tighten sanctions, extending an asset freeze to individuals and organizations helping Kim. Luxury goods were also put under sanctions.

In reality, though, by the time North Korea conducted this third test, there were few sanctions left to deploy. By 2013, the North's ability to import goods was severely limited. And Obama declined to take next steps, such as a naval blockade to block all shipments of goods. China also continued to provide the country oil and aid.

January 2016: North Korea claims to have conducted a fourth nuclear test, far underground. On state TV, Kim said the explosion came from a miniaturized hydrogen bomb and called it a “spectacular success.” Independent observers say they can't confirm that the test happened. If it did, its size is hard to measure. But they estimate that the explosive yield was between four and six kilotons.

In the months afterward, Congress passed a law empowering the administration to sanction individuals who import or export goods and money to North Korea. The United Nations passed a resolution banning North Korea from conducting “launches using ballistic missile technology.” The resolution also required all member states to “inspect cargo to/from the DPRK or brokered by the DPRK that is within or transiting their territories.” (DPRK stands for Democratic People's Republic of Korea, the country's official name.)

Obama and America's allies in the Pacific continued work on THAAD, a missile defense shield. The president rejected a deal, offered by the North Korean regime, to take down the shield in exchange for promises that the North would conduct no more nuclear tests.

September 2016: North Korea conducts a fifth nuclear test. The seismic activity generated registered 5.3 in magnitude, accompanying an explosion of about 10 kilotons. That's equivalent to the bombs dropped on Hiroshima and Nagasaki, and 10 times stronger than what the country was able to do a decade before.

In response, Obama worked with the United Nations to tighten sanctions even further.

“The United States does not, and never will, accept North Korea as a nuclear state,” he said in a speech in South Korea. “We are going to work together to make sure we're closing loopholes.” China, which has veto power on the U.N. Security Council, joined a resolution strongly condemning the missile launch. It also agreed to ban the import of North Korean coal, a major blow to that country's economy.