maybe mammals would be intelligent
Mammals are intelligent. The most intelligent being humans.
Apparently.
What if we was able to go back in time and kill the very first human
You know, it doesn't work like that. Humans didn't just spawn, there was a gradual shift from homos erectus to homo sapien. You'd have to eliminate the thing that evolved into the thing that evolved into the thing that ....blah blah... into what eventually became humans.
I'd be more worried about the iron curtain and the USSR
But then say you engineered events so that Lenins takeover of Russia fails, this affects the first
and second world wars. You could make a case that Barbarossa (which was only conducted with such haste cos Hitler
hated communism) and the subsequent grinds, especially Stalingrad, lost him the war. Europe wasn't in any position to stop him and he only cancelled Sea Lion (the invasion of the UK) cos he was impatient to turn on the Russians. If that hadn't had happened, the RAF themselves freely admit they were a fortnight from destruction by the end of the battle of britain, handing air superiority to the Luftwaffe and paving the way for a reverse D-Day onto the UK Mainland.
At which point the US continue the 'fight' against the Germans.
Only they won't.
This is still 1940. All this is before Pearl Harbour. The US isn't at war with anyone at this point and thus has no legal justification. FDR had been trying to get the US involved but there was no political will to get into the European War and certain large US corporations are still supplying the Wermacht with equipment (I'm looking at you Ford).
It's why there's such conspiracy around whether or not the US government ignored the warnings and allowed the Japanese to bomb Pearl Harbour so they could use it to convince Congress to support war.
Even
if they could use Pearl Harbour as justification to declare war on what is now a 'united' (Nazi controlled) Europe (Which, as long as Hitler plays smart, they won't be able to), logistically they're not invading Europe across the Atlantic.
Where are they going to land? The Germans control from Norway to North Africa, and any enormous invasion fleet is going to have a real bad time of things.
This isn't a D-Day hop across the channel under the umbrella of the USAAF/RAF. The Kreigsmarine still has most of its large surface warships, along with whatever is left of the Royal Navy. Then you have the U-Boat fleet, which even under pressure of the Royal Navy almost turned the war, never mind given utterly free reign of the Atlantic.
There's no aircraft capable of flying those distances, so aside from the US Carrier fleet the Luftwaffe is gonna have a whale of a time once the fleet gets within strike distance of the British and French coasts.
Then onto the beaches. Which haven't been extensively recce'd like they were on D-Day, haven't been plastered by the largest Air and Seaborne preparation bombardment in human history like on D-Day and aren't only running a short distance from re-supply like on D-Day and are facing (untested, I might add, as this is pre-Pearl Harbour) the most capable and battle hardened/experienced military in the world.
Now, imagine trying to sell this plan in September - December 1940 (after the Battle of Britain and Sealion) and again in December 1941-Feb 1942.
Especially when they've got a bigger and more aggressive problems in the shape of the Empire of Japan.
Likely endstate is the US and the Germans never get into hostilities. If the Germans invade Imperialist Russia (still ran by the Tsars), Hitler doesn't take place names personally and thus doesn't make the mistakes at Stalin/Leningrad and defeats the Russians before winter and the US deals with and eventually defeats Japan.
Germany started the space age with the V2 and NASA hired Von Braun, who helped them land on the moon. But if that changes, then the moon landing probably wouldn't have happened
I have a better one. Von Braun designed the Saturn V for the Germans and landed a man on the moon in 1947 with a Nazi victory
People put waaaaay too much emphasis on Von Braun as if he dragged the Germany/America kicking and screaming into LLO. The Space Age was well on before then, and the main thing that hindered the US was it got in the game way to late after humiliating and resource starving its greatest thinkers in that field for decades prior to Operation Paperclip/Apollo. If the US Air Force had applied funding for it, and the New York Times hadn't publicly ruined him, Goddard would've got the US to the moon. Not with a Saturn V, but something else and the time frames wouldn't have been much different.
Space Exploration isn't dependant on brilliant thinkers, it's dependant on financial backing. Von Braun didn't kickstart the Space Age to go to the moon, he convinced Hitler that these rockets could do some damage.
It was only this reason he was given the resources and during testing this wonder weapon, he managed to further our understanding of liquid fuel engines, guidance, supersonic airflow etc.
Hitler wasn't initially interested in rocketry and is quoted in calling them over-priced artillery rounds. He certainly wasn't interested in going to the moon and anyone thinking that the Nazis were landing on the moon in 1947, world war or not, isn't thinking properly.
Even then, the US didn't pinch Von Braun to go to the moon. They took him and his team to build them rockets. As weapons.
To deliver nuclear warheads. You could make a case that thanks to the stunning inefficiency of the V2 as a conventional warhead delivery system (bang to buck, the V2 program was ruinous for the Germans and it could only throw a 1000kg payload), if it wasn't for the atomic bomb and or the growing hostility between the Americans and Russians, the US may have just allowed the Russians to round up the Pennamunde crew and shoot the lot of them for war crimes.
But I digress.
The moon landings would have happened at some point. How, when and by whom is depending on the political will and the potential gain achieved.
Actually, I have a better one. The US and USSR working together during the space race instead of against each other.
Ha, imagine that. The whole 'for all mankind' deal writ large.
If I could change one moment in history it would be WW2
What would you change? Prevent it from happening by eliminating Hitler, Red Alert style? You wouldn't need to worry about the USSR, you'd need to worry about the USGR. After the depression of '29, Germany was going one of 2 ways. Hard Left, or Hard Right. If Hitler hadn't become Chancellor or joined the NSDAP or survived WWI, the nations Communist Party would have taken control of Germany by the mid to late 30s. So you essentially have to choose your poison, but either way it's not gonna be good.