April, 1940: After the bombing raids by the entire Luftwaffe (was good weather during Axis turn), and a couple daring Parachute drops, London fell.
This is the first major event that would shape the course of the war.
This came as a complete surprise, as after researching Fafnir's forum posts, I was under the impression that he would never do a Sealion. I didn't upgrade the AA of the garrison, nor London city. Luckily, I had a few units on the island to contain the paratroopers. The Axis had to use her entire air force for many turns afterwards to support the paras, which slowed down the progress in France.
The capture of London boosted German morale to an insane level (even at the end of the war, German NM was close to 110%), which helped to create all of the invincible German units that would kill anything in sight. It also mobilized Turkey and Spain greatly to the Axis side, who both eventually joined by additional Axis efforts. Both Turkey and Spain are extremely important to the Axis cause. TBH, later when Turkey was preparing for war, I thought I could have lost the game, as the Russian southern flank became wide open.
On the other hand, this also mobilized US from 35% to 53%, and Russia 22% to 45%. These are huge numbers, and they are early mobilizations, which would have their effects snowballed turn by turn. Now Russia has more than a year to have good income every turn to research and buy units. And USA would put in insane amount of research that when war broke out for her, she is very much ready to strike back.
It's hard to judge which side benefited more from this, though I"m inclined to say the Allies got more.