Under intense German pressure, Prince Paul's government signed onto the Axis alliance in Vienna, hoping to avoid invasion. The move provoked massive public protests in Belgrade and a military coup on March 27 that overthrew the regency, prompting Hitler to order the brutal invasion of Yugoslavia just weeks later, which delayed Germany's invasion of the Soviet Union.
The signing was less a triumph than an act of desperation. Squeezed between Axis powers on every border and abandoned by the Western Allies' limited capacity to help, Yugoslavia's regime under Prince Paul calculated that alignment with Germany offered the only path to avoid outright invasion, joining Hungary, Romania, and Bulgaria in the Axis orbit.
The bet failed spectacularly. Belgrade's citizens, who overwhelmingly opposed alliance with Hitler, poured into the streets in protest, and within 48 hours military officers seized power, repudiated the pact, and installed young King Peter II. An enraged Hitler, viewing this as an intolerable betrayal, ordered the Wehrmacht to crush Yugoslavia entirely. The resulting invasion and the parallel campaign in Greece are widely credited by historians with delaying Operation Barbarossa by several crucial weeks, a delay that some argue contributed to the German army's catastrophic encounter with the Russian winter later that year.