The beginning of the Cold War between the United States and the Soviet Union in 1947 set the stage for the first “hot war” in Korea.
June 25, 2025, marks the 75th anniversary of the start of the Korean War, a war which technically continues. This date is so impactful that South Koreans refer to this war that continues to divide their homeland as the “6-2-5 War,” or simply as the numbers “6-2-5” (pronounced “yook-ee-o”). With over 1.2 million South Korean lives lost—both military and civilian—the Korean War wasn’t just a national crisis; it was a deeply personal catastrophe etched into the memory of nearly every household. In the United States, it is often referred to as “The Forgotten War,” a tragic misnomer considering over 36,000 Americans lost their lives in just three years. While less than the 58,000 Americans killed in Vietnam over 20 years, the toll reveals the striking intensity and rapid escalation of combat in Korea. But Americans and Koreans were not the only ones to experience loss, as the Korean War also served as the first real test of the United Nations as a military entity, with 14 other countries providing combat units under the United Nations Command, sustaining 3,100 dead.
The losses serve as a grave reminder of a war that offers many lessons. Just two takeaways are offered here: 1) gaps in preparedness invite aggression with the cost of restoring deterrence paid in blood, and 2) the remarkable recovery of the Republic of Korea (South Korea), underwritten by the long-term commitment of the United States, speaks to the continuing relevance of the ROK-U.S. alliance—and other alliances—in today’s volatile environment. While deterrence failed in 1950, this robust alliance has prevented the resumption of hostilities on the Korean Peninsula since. Thus, seventy-five years on, the Korean War offers the United States and its allies pertinent lessons about the role of force posture and readiness for achieving deterrence in a world featuring the same players, just at a different time.
The beginning of the Cold War between the United States and the Soviet Union in 1947 set the stage for the first “hot war” in Korea. This war would see communist-aligned North Korea (backed by Russia and China) square off against the United Nations Command-backed South Korea (de facto U.S.-led) along the 38th parallel. Prior to the invasion, the United States transferred the several army divisions that had been in Korea to Japan to oversee occupation and rebuilding efforts there. In the pre-dawn hours of June 25, 1950 North Korean forces, with Russian consent, mounted a surprise attack in which they encountered the reality of the U.S. post-World War II drawdown, which left only U.S. advisors and no combat divisions supporting South Korean forces. It took only three days for the capital of Seoul to fall, and it would take until 1 July for the first U.S. reinforcements to arrive in the form of Task Force Smith.
The next three years would see dramatic shifts in territorial control as the North Koreans forced the U.S.-led coalition into the southeastern-most city of Busan, then retreated to the Chinese border of the Yalu River, before counter-attacking back to the 38th parallel with Chinese assistance. The Forgotten War illustrates a simple—but not easy—lesson: deterrence requires forward-postured, capable forces and vigilance. Ultimately, deterrence fails when capability and will are in doubt.
Another lesson emerges here, revealing the power and potential of the U.S. alliance framework for mutual prosperity and security—prosperous nations aligned with the United States are not only better equipped to contribute to their own defense, but also to contribute to the collective security of the international system. Not only did the U.S.-led coalition restore the status quo ante bellum in terms of territory, but over time the entire weight of the U.S. instruments of national power were brought to bear, in cooperation with the Republic of Korea, to restore deterrence, establish a vibrant democracy, and transform a war-ravaged nation into the global technology and cultural powerhouse it is today.
An economic comparison between North and South Korea reinforces the compounding effects of partnership with the United States. South Korea, known as “the Miracle on the Han River,” evolved from a post-war per capita income of less than $100, to the world’s 13th largest economy. On the other hand, North Korea, despite sharing ethnic homogeneity and basic geographic position, ranks 89th. Not only is South Korea thriving economically, but it contributes globally to deterrence, security, and peace— unlike its northern neighbor, which is widely regarded as a destabilizing global pariah. South Korea provides over $1 billion annually to host 28,500 U.S. servicemembers and crossed $14 billion of annual defense exports to 12 countries in 2023. The Republic of Korea has been a reliable security exporter and global partner by contributing coalition forces to Vietnam, Afghanistan, Iraq, anti-piracy operations, and peacekeeping operations. The takeaway is clear: alliances rooted in shared values and sustained through mutual investment are not only strategic assets and in the interests of the United States, they are also engines of transformation that multiply power, boost deterrence, and shape a more stable global order.
But South Korea still balances on the edge of several knives. While the ROK-U.S. alliance enforces an armistice that secures a fragile peace in a war that has not officially ended, skirmishes continue along the 155-mile blade designated as the demilitarized zone (DMZ). Occasional provocations by a nuclear-armed North Korea threaten to slice open the seams of conflict; as of this writing, North Korea conducted at least five missile launch events in 2025, including an alleged new hypersonic missile. Not only is deterrence and security of South Korea important to the Korean Peninsula, but it also represents an island of stability in a troubled region, where South Korea sits on a broader strategic knife-edge. The strategic geography of the Korean peninsula makes it a fulcrum in the Indo-Pacific—bordering China, Russia, and Japan. The commitment of military personnel in Korea not only deters North Korean aggression and its nuclear ambitions, but also serves as a regional hedge against China and Russia, especially with the Republic of Korea’s role in regional security frameworks such as U.S.-ROK-Japan trilateral cooperation.
Is the United States postured with the right forces in the right places?
Therefore, as policymakers and military leaders reflect on the start of the Korean War 75 years ago—a fiery eruption that receded into a perpetual simmer—we can draw parallels to current events and ask questions about future scenarios. In his memoirs, President Harry Truman stated the decision to help South Korea was intended to prevent its collapse, which would, in turn, encourage “China and the Soviet Union to attack other nations, resulting in World War III.” The same logic applies today with the Russia-Ukraine war and a potential China-Taiwan conflict. The U.S. military has attempted to pivot to the Pacific for over a decade. However, many Americans still grapple with the reality of China’s rhetoric, preparation, and exercises to invade its democratic neighbor, Taiwan. Certainly, parallels can be drawn between the Korean War and Russia’s invasion of Ukraine, but we should view Russia’s ambitions in broader terms. Some agree with the opinion of Hamish de Bretton-Gordon, a retired British Army officer, that Russian President Vladimir Putin’s invasion of Ukraine “brought the Cold War back to Europe, which could at any time turn hot.” Other experts, such as former deputy national security advisor Elliott Abrams, take a broader perspective, arguing that a “new Cold War” has been “brought upon the West by the aggressive partnership of Xi Jinping and Vladimir Putin.” Yet others, such as historian Philip Zelikow, imply we are potentially in the opening phases of World War III. If this is the case, then we may be where we started prior to the Korean War, and would be wise to consider This Kind of War, where T.R. Fehrenbach surmises,
Because the American people have traditionally taken a warlike, but not military, attitude to battle, and because they have always coupled a certain belligerence—no American likes being pushed around—with a complete unwillingness to prepare for combat, the Korean War was difficult, perhaps the most difficult in their history.
These words by Fehrenbach cut like a knife, one that remains sharp if we were to use them to examine America’s current preparedness for war. Is the United States postured with the right forces in the right places? Are decision-makers and military leaders vigilant and clear-eyed on the prospects for conflict with Russia and China? And finally, is the American public ready to stomach the costs of war if it is necessary?
While not the case on June 25, 1950, the U.S.-ROK alliance today serves as an excellent example of deterrence through forward posture, capable forces, and vigilance. The United States should strengthen this and other key alliances to promote economic strength, political stability, and technological advancement, which enables U.S. allies to build capable, professional militaries; invest in interoperable defense systems; and participate meaningfully in joint operations and peacekeeping. The U.S. alliance system thus becomes more than the sum of its individual parts—instead it evolves into a tightly woven mesh of burden-sharing states that reinforce regional stability, respond to crises, and reinforce the rules-based international order. In the end America and its allies can reinforce the shield of deterrence with forward postured, capable forces, possessing the will and capability to defend our interests and prove the lessons of the Korean War are not forgotten.
Andy Bogusky is an Air Force colonel and a Faculty Instructor in the Department of Military Strategy, Planning and Operations at the U.S. Army War College. A B-52 and MQ-9 pilot, he is the President of the Pennsylvania Chapter of the Korea Defense Veterans Association.
John Nagl is Professor of Warfighting Studies at the U.S. Army War College. He is the author of Learning to Eat Soup with a Knife: Counterinsurgency Lessons from Malaya and Vietnam.
The views expressed in this article are those of the author and do not necessarily reflect those of the U.S. Army War College, the U.S. Army, or the Department of Defense.
Photo Description: The Korean War Veterans Memorial in Washington, D.C., honors the sacrifices of those who served in the Korean War (1950-1953). It features 19 stainless steel statues of soldiers, a mural wall with etched faces, and a Pool of Remembrance.
Photo Credit: Pixabay