OSAN AIR BASE, South Korea — With the stakes on the Korean peninsula as high as they have ever been, life for the men and women here with the U.S. Air Force's 51st Fighter Wing is lived in a state of constant alert. 
 
 South Korea has no nuclear weapons, so it relies on the United States' nuclear umbrella to deter North Korea and its erratic leader, Kim Jong Un. 
 
 That makes Osan — less than 50 miles from the Demilitarized Zone separating the mortal enemies and home to a large proportion of the 28,500 U.S. military personnel in South Korea — the leading edge of the shield that protects the South from the North. 
 
 Hence the base's slogan: "Ready to Fight Tonight." 
 
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 NBC Nightly News Exclusive: Lester Holt anchors from South Korea on Monday and Tuesday, April 3 and 4, at 6:30pm ET. 
 
 Patriot missile batteries are poised to defend the base from North Korean ballistic missiles. Preparations for chemical attacks are constant. Air crews hone their combat readiness round the clock. 
 
 "We need to be ready every day to not only fight tonight, but win tonight," Lt. Col. Mike McCarthy, an F-16 pilot, told NBC News, which was granted rare access to the underground Air Operations Center. 
 
 Following a U-2 spy plane preparing for takeoff at daybreak from Osan Air Base in S Korea. My report Monday night on @NBCNightlyNews pic.twitter.com/9a231HVOaw — Lester Holt (@LesterHoltNBC) April 2, 2017 
 
 It's a facility so critical that war planners believe it's the North's No. 1 target should hostilities break out, and the military wouldn't allow NBC News to use the full names of front-line forces. 
 
 Related: U.S. Begins Shipping Controversial Anti-Missile System to South Korea 
 
 As a pilot identified only as Capt. Mike embarked on an eight-hour reconnaissance mission in a U-2 spy plane, a lieutenant colonel named Dane said: "It's a constant tempo. We are very busy all the time, and I think in the last 10 years we've been the busiest we have ever been in this airplane." 
 
 That's because, as Navy Adm. Scott Swift, commander of the U.S. Pacific Fleet, put it, "I have never seen a greater concern from a regional perspective of the threat that is being posed by North Korea." 
 
 Air Force Lt. Col. Michael McCarthy talked to NBC News' Lester Holt. NBC News 
 
 Kim is "desperate" and is prepared to use nuclear weapons to strike the United States and its allies, Thae Yong Ho, the highest-profile North Korean defector in two decades, told NBC News. 
 
 Related: North Korean Defector Tells Lester Holt 'World Should Be Ready' 
 
 That means that any moment, the men and women of Osan could have to drop everything, grab their gear and go into battle, said Col. Andrew Hansen, commander of the 51st Fighter Wing. 
 
 "In terms of the constant day-to-day threat ... it is certainly more dangerous in Iraq and Afghanistan," Hansen said. "But in terms of the dominance of the threat — the capability of North Korea — certainly this is a much more dangerous place." ||||| An Air Force WC-135 Constant Phoenix, which is commonly referred to as a nuke-sniffer, arrived at Kadena Air Base, Japan, Friday, April 7, 2017, said an Okinawan man who monitors military aircraft traffic at the installation. 
 
 CAMP FOSTER, Okinawa — A U.S. aircraft that specializes in detecting radioactive debris after the detonation of a nuclear device has arrived on Okinawa amid rising tensions on the Korean peninsula. 
 
 An Air Force WC-135 Constant Phoenix — commonly referred to as a nuke-sniffer — arrived at Kadena Air Base last Friday evening, said Satoru Kuba, an Okinawan who monitors military aircraft traffic at Kadena Air Base. 
 
 Photos obtained by Stars and Stripes show the plane sitting on the base’s runway. 
 
 A spokesperson for the Air Force’s 18th Wing at Kadena did not immediately respond to requests for comment, and Japan’s Ministry of Defense would not immediately verify the report. 
 
 However, a senior Japan Self-Defense Forces official confirmed the aircraft’s arrival, according to a report Wednesday by Japan’s Nikkei newspaper. 
 
 The Pentagon has often deployed one of the Air Force’s two WC-135 aircraft to the Asia-Pacific region since North Korea detonated an underground nuclear device in 2006. The plane also flew over Japan in 2011 after the meltdown at the Fukushima Dai-ichi nuclear power plant, according to the Washington Post. 
 
 The modified C-135B uses external flow-through devices that collect air samples and debris. The samples later go to a lab for analysis. 
 
 The Constant Phoenix arrives at a time of growing tensions with North Korea, which has conducted two nuclear tests and test-fired nearly 30 missiles since last year. 
 
 Acting South Korean President Hwang Kyo-ahn has warned that the North may conduct “provocations” in connection with several upcoming events. 
 
 On Saturday, Pyongyang will celebrate the 105th birthday of Kim Il Sung, its late founder and grandfather of current leader Kim Jong Un. On April 25, the communist state will mark the 85th anniversary of its Korean People’s Army. In May, South Korea will hold presidential elections. 
 
 In reaction to last week’s U.S. missile launches against Syria, North Korea warned of “catastrophic consequences” if the United States attempts a pre-emptive strike. 
 
 The tensions led Pacific Command to order the USS Carl Vinson strike group toward waters near the Korean peninsula for the second time in recent months, rather than onward to Australia for planned port visits. 
 
 President Donald Trump’s administration has signaled it will take a hardline stance against Pyongyang’s nuclear-weapons program and has called on further action from China to rein in its communist ally. 
 
 Chinese President Xi Jinping told Trump this week he would work with the U.S. to denuclearize the peninsula, but that it must be done peacefully. 
 
 The Constant Phoenix that arrived on Okinawa was at England’s Royal Air Force Mildenhall in March, according to photos of its tail numbers. 
 
 Air Force officials told Stars and Stripes in March the aircraft was on a scheduled routine mission and refuted media speculation that it was investigating a Russian nuclear test. 
 
 The Constant Phoenix next emerged in March when it had engine trouble on its way to Kadena and was forced to make an emergency landing in Banda Aceh, Indonesia. A spokesman from the Indonesian air force told The Associated Press the plane requested permission to land because one of its four engines had failed. 
 
 It was carrying 20 American military personnel from Diego Garcia in the central Indian Ocean, the spokesman told AP. The cause of the failure was unclear and no one was injured in the landing. 
 
 burke.matt@stripes.com 
 
 sumida.chiyomi@stripes.com ||||| A U.S. Air Force F-16 fighter jet lands on the runway at the Osan U.S. Air Base in Pyeongtaek, South Korea, Thursday, Aug. 10, 2017. North Korea on Wednesday officially dismissed President Donald Trump's... (Associated Press) 
 
 SEOUL, South Korea (AP) — North Korea has announced a detailed plan to launch a salvo of ballistic missiles toward the U.S. Pacific territory of Guam, a major military hub and home to U.S. bombers. If carried out, it would be the North's most provocative missile launch to date. 
 
 The announcement Thursday warned that the North is finalizing a plan to fire four of its Hwasong-12 missiles over Japan and into waters around the tiny island, which hosts 7,000 U.S. military personnel on two main bases and has a population of 160,000. 
 
 Japan and South Korea vowed a strong reaction if the North were to go through with the plan. 
 
 It said the plan, which involves the missiles hitting waters 30 to 40 kilometers (19 to 25 miles) from the island, could be sent to leader Kim Jong Un for approval within a week or so. It would be up to Kim whether the move is actually carried out. 
 
 It is unclear whether — or exactly why — North Korea would risk firing missiles so close to U.S. territory. Such a launch would almost compel the United States to attempt an intercept and possibly generate further escalation. 
 
 North Korea, no stranger to bluffing, frequently uses extremely bellicose rhetoric with warnings of military action to keep its adversaries on their heels. It generally couches its threats with language stating it will not attack the United States unless it has been attacked first or has determined an attack is imminent. 
 
 But the statement raised worries amid threats from both sides. 
 
 Following reports that U.S. intelligence suggests the North might be able to pair a nuclear warhead with a missile capable of reaching targets on the United States mainland, Trump warned North Korea that "it faces retaliation with fire and fury unlike any the world has seen before." 
 
 Pyongyang, meanwhile, has been louder in its complaints against a new and tough round of sanctions imposed by the United Nations, with strong U.S. backing, and Washington's use of Guam as a staging ground for its stealth bombers, which could be used to attack North Korea and are a particularly sore point with the ruling regime in Pyongyang. 
 
 Its reported plan is extremely specific, suggesting it is actually plotting a launch. 
 
 The report said the Hwasong-12 rockets would fly over Shimane, Hiroshima and Koichi prefectures in Japan and travel "1,065 seconds before hitting the waters 30 to 40 kilometers away from Guam." It said the Korean People's Army Strategic Force will finalize the plan by mid-August, present it to Kim Jong Un and "wait for his order." 
 
 "We keep closely watching the speech and behavior of the U.S.," it said. 
 
 Such a move would not merely be a test launch, but a demonstration of military capabilities that could easily lead to severe consequences. 
 
 South Korea's military responded by saying North Korea will face a "stern and strong" response from Washington and Seoul. Taking it a step further, Japan's Defense Minister Itsunori Onodera told parliament a missile attack on the U.S. territory would be a Japanese national emergency because it would threaten Japan's existence as a nation. 
 
 If North Korea were to actually carry it out — even if it aimed at hitting the waters off Guam and not the island itself — that would clearly pose a potential threat to U.S. territory and put the United States in a much more complicated situation than it has been during previous missile launches. 
 
 Guam lies about 2,100 miles (3,400 kilometers) from the Korean Peninsula, and it's extremely unlikely Kim's government would risk annihilation with a pre-emptive attack on U.S. citizens. It's also unclear how reliable North Korea's missiles would be against such a distant target, but no one was writing off the danger completely. 
 
 Washington has been testing its missile defenses in response to the North's stepped-up development and the current escalation of tensions could lead to pressure for the U.S. military to try to shoot down the North's missiles in midflight if they are heading toward Guam. 
 
 That would likely open up a set of very major problems, including the possibility of both a very high-profile failure or a miscalculation of Washington's intentions and a more deadly pre-emptive strike by the North — which has missiles able to hit Tokyo and conventional weapons that could devastate South Korea's capital of Seoul. 
 
 The Hwasong-12, which was revealed for the first time at a military parade in April, is an intermediate-range ballistic missile that is believed to have a radius of more than 3,700 kilometers (2,300 miles). It can be fired from mobile launchers, making it hard to detect and destroy on the ground. 
 
 By launching a salvo of four, the North would be attempting to make it harder for the U.S. to intercept all of the incoming missiles. Its stated flight path over Japan is also very aggressive — it has recently tried to avoid flying over neighboring countries by shooting its missiles up at a very high angle to land in the ocean. 
 
 Washington, meanwhile, has been giving out mixed signals about its intentions. 
 
 While Trump was threatening annihilation and boasting from the New Jersey golf resort where he's vacationing that he has made the U.S. nuclear arsenal "far stronger and more powerful than ever before," Secretary of State Rex Tillerson sought to calm the sense of crisis. 
 
 "Americans should sleep well at night," Tillerson told reporters. "Nothing that I have seen and nothing that I know of would indicate that the situation has dramatically changed in the last 24 hours."
– Osan Air Base may not be a name you're familiar with, but it's a US Air Force base that sits in a prime location: fewer than 50 miles from the Demilitarized Zone that separates North and South Korea. If that's not enough to get your attention, this line from NBC News might change that: "It's a facility so critical that war planners believe it's the North's No. 1 target should hostilities break out," and thousands of US military personnel are stationed there. The network reports on its exclusive visit to the base, which is home to the US Air Force's 51st Fighter Wing. There, air crews and patriot missile batteries are at the ready in the event of ballistic missiles or chemical attacks. When North Korea launched four ballistic missiles toward Japan in March, the Washington Post noted the launch site was fewer than 300 miles from the Osan base. An F-16 pilot describes the state of constant alert as not just readiness, but the ability to "win tonight." Thae Yong Ho, a recent high-profile defector, tells the network that North Korea's Kim Jong Un is "desperate" and prepared to use nuclear force, and a lieutenant colonel at Osan says it's "the busiest we have ever been in this airplane," referring to a U-2 spy plane. Meanwhile, the US Air Force reported in a press release on a two-week command and control exercise held across the Korean Peninsula in March that saw 12,800 US forces join 10,000 South Korean military personnel and others from Australia, Canada, Denmark, France and Great Britain. It took place just as the US sent part of a controversial missile defense system called THAAD to Osan, reports NPR. (Japan weighs in on this week's "extremely problematic" test.)