INDIANAPOLIS (WISH) — Eighty years ago was one of the most crucial moments in world history.

D-Day was critical in turning the tides in World War II for the Allied forces.

Troops stormed the beaches of Normandy, France, and claimed victory over Nazi Germany. In the lead-up to the event, meteorologists were tasked with one of the most important forecasts ever which proved to be very challenging.

A strong system approaching Europe on June 4, 1944, ended up delaying the original invasion of northern France on June 5. There were even disagreements in the forecast between American and British forecasters. Ultimately, Group Capt, James Stagg of the United Kingdom’s Supreme Headquarters, Allied Expeditionary Force, was the one who recommended the invasion take place on June 6 based on weather observations from a ship in the Atlantic.

On June 5, 1944, low-level clouds and winds over 25 mph consistently set up in northern France along a cold front. If the Allies had gone forward with the original plan, the invasion would have been extremely difficult in those conditions.

The weather on the day of the invasion was not perfect, but it was enough for the invasion to happen. German meteorologists did not have the surface observations the Allied forces had. So, they did not anticipate a break in the weather on D-Day to aid in the surprise of the attack.

After June 6, multiple storm systems would lash western Europe. Who knows if D-Day would have even happened if it were delayed any further.