Seventy years ago today, an American B-29 bomber dropped an atomic bomb over the Japanese city of Hiroshima. The massive blast, equivalent to about 13,000 tonnes of TNT, killed 80,000 people, or 30 percent of the population, immediately or within hours and laid waste to much of the city. Three days later, on August 9, 1945, the US unleashed another atomic weapon on the city of Nagasaki, killing another 40,000 people outright.

Many more people died subsequently of their injuries, including from radiation sickness. Estimates of the total number of men, women and children killed by the two bombs ranged from 200,000 to 350,000, just in the first four months. In the years that followed, more died from leukaemia and other cancers as a result of exposure to intense radiation. For those who survived, the horrific scenes of the dead and dying left deep psychological scars.

Washington’s unleashing of atomic weapons against civilian populations was a criminal act of the first order, shattering forever the myth that the United States was a force for democracy and decency. US imperialism pursued its war aims against Japan to ensure its dominance in Asia with the same ruthlessness and contempt for human life as its Japanese rival. With the bombing of Hiroshima and Nagasaki, the US announced to the world its bid for global hegemony in the post-World War II era.

The scale of these atrocities is paralleled by the magnitude of the lies used to defend them. While Hiroshima and Nagasaki each had some military facilities and industry, the use of such an indiscriminate weapon of mass destruction was designed to “shock and awe” not just the Japanese people but the world.

The main justification adopted by US President Harry Truman, echoed repeatedly right down to today, was that the atom bombs were dropped to “save lives.” By forcing Japan’s immediate surrender, it is claimed, the incineration of the two cities averted an American invasion of Japan that would have resulted in many more American and Japanese deaths.

Every aspect of the argument is flawed or false. The estimates of the death toll from a US invasion were deliberately inflated to make the case for using the atomic weapons. The Truman administration rejected proposals made by some scientists working on the bomb that its destructive capacity should be demonstrated to the Japanese regime by dropping it on an uninhabited area.

Moreover, Tokyo had already put out peace feelers. Its navy and air force were largely destroyed and much of its industry was shattered by relentless American bombing. The US had demonstrated its capacity to level Japanese cities through the use of incendiary devices designed to trigger fire storms. The fire-bombing of Tokyo in March 1945—itself a terrible war crime—resulted in the deaths of an estimated 87,000 people in one night.

The Potsdam conference in July 1945, involving the US, Britain and the Soviet Union, had issued an ultimatum to Japan of “unconditional surrender.” Following the bombing of Hiroshima, the final straw for Tokyo was the Soviet Union’s entry into the Pacific War on August 8 and its invasion of Japanese-occupied Manchuria. Truman’s decision to unleash a second atomic weapon against Nagasaki—a day later—was motivated by Washington’s determination to ensure that the US presided over Japan’s surrender, which came on August 15 with Emperor Hirohito’s address to the nation.

The American use of the atomic weapons sought to terrorise not just the Japanese regime, but above all the Soviet Union, and ensure post-war US global dominance. The Truman administration rejected the proposal for targetting an uninhabited area because it wanted to demonstrate to the world, not only the immense destruction an atom bomb could cause, but Washington’s willingness to use it against civilian populations.

Seventy years later, geo-political tensions and the danger of a Third World War are rapidly rising amid the deepening economic breakdown that has followed the 2008 global financial crisis. The fundamental contradictions of capitalism—between world economy and the outmoded nation state system, and between socialised production and the private ownership of the means of production—that led to two world wars in the twentieth century are creating the conditions for another global conflagration.

The most destabilising factor in world politics today is the United States. Since the collapse of the Soviet Union in 1991, Washington has repeatedly resorted to military might, including in the Middle East, the Balkans and Central Asia, in a bid to offset its economic decline. In the past year, Washington’s intrigues and interventions have assumed an ever-more reckless character, directed in particular against China and Russia.

Last year’s fascist coup in Ukraine, engineered by the US and Germany, has been followed by a provocative military NATO build-up in Eastern Europe against Russia that has greatly heightened the risk of conflict between nuclear-armed powers. On the other side of Eurasia, the US “pivot to Asia” has dangerously inflamed regional flashpoints, such as the South China Sea, that could trigger a war between the US and China.

All the major imperialist powers are preparing for war. Germany and Japan are rapidly throwing off the post-war constraints on their armed forces and remilitarising. While at present operating within the framework of a US alliance, both German and Japanese imperialism have economic and strategic interests that could put them at odds with Washington. It should be recalled that the last war between the US and Japan was fought over which power was going to dominate China and Asia.

The Second World War ended with the dropping of atomic bombs. A third would inevitably involve the use of nuclear weapons that will dwarf those used on Hiroshima and Nagasaki. US imperialism’s determination to maintain and augment its nuclear supremacy is underlined by its plans to invest $1 trillion over the next 30 years in upgrading its huge arsenal of nuclear weapons and delivery systems.

The lesson to be drawn from the atomic bombs dropped on Japan 70 years ago is that the US, and indeed all the imperialist powers, will stop at nothing—even if it threatens the survival of humanity—in the ruthless prosecution of their interests. The only social force that is capable of preventing the slide into a nuclear catastrophe is the international working class, through a revolutionary struggle to abolish the profit system. That is the significance of the campaign being waged by the International Committee of the Fourth International and its sections around the world to build an international anti-war movement of workers based on socialist internationalism.