Japan breaks from military tradition to protect American ship

Japan is lending a hand to the U.S. Navy as it tries to pressure North Korea.

The Japanese navy is sending its largest warship, the Izumo, to escort a U.S. supply ship heading toward the Korean Peninsula. That U.S. ship is refueling a fleet in the area.

The Izumo is only protecting the American ship within Japanese waters, but it's still a big break from tradition for the nation's military.

SEE MORE: How World Leaders Are Responding To North Korean Aggression

Since the end of WWII, Japan has only been able to act in self-defense. This is the first military movement under a new Japanese law that allows its ships to come to the defense of an ally that's protecting Japan.

Related: See inside the internment of Japanese citizens during WWII:

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Japanese internment camps
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Japanese internment camps
Toyo Miyatake stands in his children's bedroom looking at his young daughter drawing at a desk, while her mother stands behind her, at the Manzanar War Relocation Center in California, in this 1943 handout photo. February 19, 2017 marks the 75th anniversary of FDR signing executive order 9066, authorizing the internment of Japanese Americans during the Second World War. Courtesy Ansel Adams/Library of Congress, Prints and Photographs Division, LC-A35-4-M-10/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
A marble monument with an inscription that reads, "Monument for the Pacification of Spirits," in the cemetary at the Manzanar War Relocation Center in California, in this 1943 handout photo. February 19, 2017 marks the 75th anniversary of FDR signing executive order 9066, authorizing the internment of Japanese Americans during the Second World War. Courtesy Ansel Adams/Library of Congress, Prints and Photographs Division, LC-A35-4-M-10/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
A mess line is formed in front of a building at midday at the Manzanar War Relocation Center in California, in this 1943 handout photo. February 19, 2017 marks the 75th anniversary of FDR signing executive order 9066, authorizing the internment of Japanese Americans during the Second World War. Courtesy Ansel Adams/Library of Congress, Prints and Photographs Division, LC-A35-4-M-10/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
A terra cotta frieze by artist Steve Gardner, depicting a Japanese American strawberry farmer, is seen at the Japanese American Exclusion Memorial on Puget Sound's Bainbridge Island, Washington, U.S. February 12, 2017. The island's Japanese-American community was the first to be sent to World War Two internment camps after President Franklin D. Roosevelt signed executive order 9066 75 years ago, on February 19, 1942. Picture taken February 12, 2017. REUTERS/Chris Helgren EDITORIAL USE ONLY
Mrs. Ryie Yoshizawa and a class of female students sit at a table looking at fashion magazines and patterns. The students are: Satoko Oka, Chizuko Karnii, Takako Nakanishi, Kikiyo Yamasuchi, Masako Kimochita, Mitsugo Fugi, Mie Mio, Chiye Kawase, and Miyeko Hoshozike, at the Manzanar War Relocation Center in California, in this 1943 handout photo. February 19, 2017 marks the 75th anniversary of FDR signing executive order 9066, authorizing the internment of Japanese Americans during the Second World War. Courtesy Ansel Adams/Library of Congress, Prints and Photographs Division, LC-A35-4-M-10/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
A nurse tends to four infants in cribs at the Manzanar War Relocation Center in California, in this 1943 handout photo. February 19, 2017 marks the 75th anniversary of FDR signing executive order 9066, authorizing the internment of Japanese Americans during the Second World War. Courtesy Ansel Adams/Library of Congress, Prints and Photographs Division, LC-A35-4-M-10/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
Benji Iguchi driving tractor in a field at the Manzanar War Relocation Center in California, in this 1943 handout photo. February 19, 2017 marks the 75th anniversary of FDR signing executive order 9066, authorizing the internment of Japanese Americans during the Second World War. Courtesy Ansel Adams/Library of Congress, Prints and Photographs Division, LC-A35-4-M-10/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY.
Origami cranes hang near a terra cotta frieze, by artist Steve Gardner, depicting a separated family at the Japanese American Exclusion Memorial on Puget Sound's Bainbridge Island, Washington, U.S. February 12, 2017. The island's Japanese-American community was the first to be sent to World War Two internment camps after President Franklin D. Roosevelt signed executive order 9066 75 years ago, on February 19, 1942. Picture taken February 12, 2017. REUTERS/Chris Helgren EDITORIAL USE ONLY
Mrs. Yaeko Nakamura, holding hands with her two daughters, Joyce Yuki Nakamura and Louise Tami Nakamura, walk under a pavilion in a park at the Manzanar War Relocation Center in California, in this 1943 handout photo. February 19, 2017 marks the 75th anniversary of FDR signing executive order 9066, authorizing the internment of Japanese Americans during the Second World War. Courtesy Ansel Adams/Library of Congress, Prints and Photographs Division, LC-A35-4-M-10/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY
Ferries dock under mountain peaks of the distant Olympic Peninsula, on Puget Sound's Bainbridge Island, Washington, U.S. February 12, 2017. The island's Japanese-American community was the first to be sent to World War Two internment camps after President Franklin D. Roosevelt signed executive order 9066 75 years ago, on February 19, 1942. Picture taken February 12, 2017. REUTERS/Chris Helgren
A guard tower at Manzanar internment camp is seen in Independence, California July 17, 2013. Nearly 120,000 people of Japanese ancestry were removed from their homes on the west coast by the U.S. Army and sent to Manzanar and nine other internment camps between March 1942 and November 1945. Two thirds of them were American citizens. REUTERS/Lucy Nicholson (UNITED STATES - Tags: SOCIETY)
A view out of the window of the former living quarters at Manzanar internment camp is seen in Independence, California July 17, 2013. Nearly 120,000 people of Japanese ancestry were removed from their homes on the west coast by the U.S. Army and sent to Manzanar and nine other internment camps between March 1942 and November 1945. Two thirds of them were American citizens. REUTERS/Lucy Nicholson (UNITED STATES - Tags: SOCIETY)
The cemetery at Manzanar internment camp is seen in Independence, California July 17, 2013. Nearly 120,000 people of Japanese ancestry were removed from their homes on the west coast by the U.S. Army and sent to Manzanar and nine other internment camps between March 1942 and November 1945. Two thirds of them were American citizens. REUTERS/Lucy Nicholson (UNITED STATES - Tags: SOCIETY)
The former living quarters at Manzanar internment camp are seen in Independence, California July 17, 2013. Nearly 120,000 people of Japanese ancestry were removed from their homes on the west coast by the U.S. Army and sent to Manzanar and nine other internment camps between March 1942 and November 1945. Two thirds of them were American citizens. REUTERS/Lucy Nicholson (UNITED STATES - Tags: SOCIETY)
A gravestone is seen in the cemetery at Manzanar internment camp in Independence, California July 17, 2013. Nearly 120,000 people of Japanese ancestry were removed from their homes on the west coast by the U.S. Army and sent to Manzanar and nine other internment camps between March 1942 and November 1945. Two thirds of them were American citizens. REUTERS/Lucy Nicholson (UNITED STATES - Tags: SOCIETY)
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American ships were sent to the Korean Peninsula to act as a deterrent. But the North launched another failed ballistic missile test on Saturday –– its fourth since March.

Japan's prime minister called the latest missile launch a "grave threat" to his country.

See more on North Korea:

20 PHOTOS
North Korea unveils new weapons at military parade
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North Korea unveils new weapons at military parade
North Korean leader Kim Jong Un applauds during a military parade marking the 105th birth anniversary of the country's founding father, Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj
Missiles are driven past the stand with North Korean leader Kim Jong Un and other high ranking officials during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj TPX IMAGES OF THE DAY
High ranking military officers cheer as North Korean leader Kim Jong Un arrives for a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj
People react as they march past the stand with North Korean leader Kim Jong Un during a military parade marking the 105th birth anniversary of the country's founding father Kim Il Sung, in Pyongyang, April 15, 2017. REUTERS/Damir Sagolj
TOPSHOT - Korean People's Army (KPA) tanks are displayed during a military parade marking the 105th anniversary of the birth of late North Korean leader Kim Il-Sung in Pyongyang on April 15, 2017. North Korean leader Kim Jong-Un on April 15 saluted as ranks of goose-stepping soldiers followed by tanks and other military hardware paraded in Pyongyang for a show of strength with tensions mounting over his nuclear ambitions. / AFP PHOTO / Ed JONES (Photo credit should read ED JONES/AFP/Getty Images)
Korean People's Army (KPA) soldiers march on Kim Il-Sung squure during a military parade marking the 105th anniversary of the birth of late North Korean leader Kim Il-Sung in Pyongyang on April 15, 2017. North Korean leader Kim Jong-Un on April 15 saluted as ranks of goose-stepping soldiers followed by tanks and other military hardware paraded in Pyongyang for a show of strength with tensions mounting over his nuclear ambitions. / AFP PHOTO / Ed JONES (Photo credit should read ED JONES/AFP/Getty Images)
North Korean leader Kim Jong Un waves to people attending a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj
Military vehicles carry missiles with characters reading "Pukkuksong" during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj
Members of the Korean People's Army (KPA) ride on mobile missile launchers during a military parade marking the 105th anniversary of the birth of late North Korean leader Kim Il-Sung in Pyongyang on April 15, 2017. North Korean leader Kim Jong-Un on April 15 saluted as ranks of goose-stepping soldiers followed by tanks and other military hardware paraded in Pyongyang for a show of strength with tensions mounting over his nuclear ambitions. / AFP PHOTO / Ed JONES (Photo credit should read ED JONES/AFP/Getty Images)
North Korean soldiers march and shout slogans during a military parade marking the 105th birth anniversary of the country's founding father Kim Il Sung in Pyongyang, North Korea, April 15, 2017. REUTERS/Damir Sagolj
An unidentified rocket is displayed during a military parade marking the 105th anniversary of the birth of late North Korean leader Kim Il-Sung in Pyongyang on April 15, 2017. North Korean leader Kim Jong-Un on April 15 saluted as ranks of goose-stepping soldiers followed by tanks and other military hardware paraded in Pyongyang for a show of strength with tensions mounting over his nuclear ambitions. / AFP PHOTO / Ed JONES (Photo credit should read ED JONES/AFP/Getty Images)
People carry flags in front of statues of North Korea founder Kim Il Sung (L) and late leader Kim Jong Il during a military parade marking the 105th birth anniversary Kim Il Sung in Pyongyang, April 15, 2017. REUTERS/Damir Sagolj TPX IMAGES OF THE DAY
North Korean soldiers march and shout slogans during a military parade marking the 105th birth anniversary of country's founding father, Kim Il Sung in Pyongyang, North Korea April 15, 2017. REUTERS/Damir Sagolj TPX IMAGES OF THE DAY
A soldier salutes from atop an armoured vehicle as it drives past the stand with North Korean leader Kim Jong Un during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj
North Korean soldiers march and shout slogans during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung in Pyongyang, North Korea, April 15, 2017. REUTERS/Damir Sagolj
North Korean soldiers attend a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung in Pyongyang, North Korea, April 15, 2017. REUTERS/Damir Sagolj
Attendees carry sheets in colours of the national flag of North Korea during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung, in Pyongyang April 15, 2017. REUTERS/Damir Sagolj
North Korean soldiers, some of them on horses, march during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung in Pyongyang, North Korea, April 15, 2017. REUTERS/Damir Sagolj
Civilian attendees watch North Korean soldiers marching during a military parade marking the 105th birth anniversary of country's founding father Kim Il Sung in Pyongyang, North Korea, April 15, 2017. REUTERS/Damir Sagolj
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