On Sept. 1, 1939, one week after Nazi Germany and the Soviet Union signed a non-aggression pact, more than a million German troops—along with 50,000 Slovakian soldiers—invaded Poland. Two weeks later, a half-million Russian troops attacked Poland from the east. After years of vague rumblings, explicit threats and open conjecture about the likelihood of a global conflict—in Europe, the Pacific and beyond—the Second World War had begun.
The ostensible aim of Germany's unprovoked assault, as publicly stated by Hitler and other prominent Nazi officials, was the pursuit of lebensraum—that is, territory deemed necessary for the expansion and survival of the Reich. But, of course, Hitler had no intention of ending his aggression at Poland's borders, and instead was launching a full-blown war against all of Europe. (On Sept. 3, both England and France declared war on Germany—but not on the USSR.)
The invasion—during which German troops, especially, drew virtually no distinction between civilians and military personnel and routinely attacked unarmed men, women and children—lasted just over a month. Caught between two massive, well-armed powers, the Polish army and its Air Force fought valiantly (contrary to legend, which has the Poles surrendering quickly, with barely a whimper). In the end, Poland's soldiers and aviators, fighting on two fronts, were simply overwhelmed.
In the weeks and months after the invasion, a German photographer named Hugo Jaeger traveled extensively throughout the vanquished country, making color pictures of the chaos and destruction that the five-week battle left in its wake. Here, on the 75th anniversary of the start of World War II, LIFE.com presents a series of Jaeger's pictures from Poland: portraits of a country subjugated not by one enemy, but by several.
In Jaeger's photos, meanwhile, we see early, unsettling evidence of the violence, unprecedented in its scope, that would soon be visited upon scores of countries and countless people around the globe, from the streets of London and the forests of Belgium to the North African desert and the sun-scorched islands of the South Pacific.