Blowback is an unpleasant, unintended consequence of a military or paramilitary operation. The operation is often covert and the consequence usually only manifests itself over a significant time-period so that what seemed like a good idea at the time is revealed in retrospect, and in the long-view of history, as profoundly damaging.
An example often quoted is America's support for the Islamic extremist militants who fought against the Soviet Union following its invasion of Afghanistan. America and its allies provided funding, training and equipment for the jihadists who travelled from all over the world to wage war on the hated Soviets. When the Soviets withdrew in 1989, and the puppet regime they had left in place was defeated a few years later, an intolerant regime of religious extremists, the Taliban, was able to take power. In addition, militants from all over the world had received vital training and experience in the almost decade-long conflict, assets they were able to later put to deadly use, against America itself, in other parts of the world.
An example often quoted is America's support for the Islamic extremist militants who fought against the Soviet Union following its invasion of Afghanistan. America and its allies provided funding, training and equipment for the jihadists who travelled from all over the world to wage war on the hated Soviets. When the Soviets withdrew in 1989, and the puppet regime they had left in place was defeated a few years later, an intolerant regime of religious extremists, the Taliban, was able to take power. In addition, militants from all over the world had received vital training and experience in the almost decade-long conflict, assets they were able to later put to deadly use, against America itself, in other parts of the world.