Final answer:
The speech by Winston Churchill in 1946 where he declared that an 'iron curtain' had descended across Europe is widely considered the event that heralded the onset of the Cold War.
Step-by-step explanation:
In March 1946, the event signaling the onset of what would become the Cold War occurred during a speech by Winston Churchill in Fulton, Missouri. Churchill proclaimed that an 'iron curtain' had descended across the continent, marking the division between the Western countries allied with the United States and Eastern European nations under Soviet Union dominance.
This ideological and geopolitical bifurcation fueled a nearly five-decade standoff, characterized by an arms race, proxy wars, and a competition in technological and scientific advancements such as the space race.
While the year 1946 was indeed pivotal, the tensions between the US-led Western bloc and the Soviet Union began to swell immediately after World War II. These tensions were exacerbated by various political clashes and escalations such as the Berlin blockade, the Korean War, and Cuban Missile Crisis, which brought the world perilously close to nuclear war. Though direct military confrontation between the US and USSR was avoided, the era was rife with global political turmoil and conflict, influencing international relations for generations.