The Cuban missile crisis

Japan attacked Pearl Harbor because the U.S. imposed an economic embargo in response to Japan’s invasion of China. The U.S. ended up in the war because they demanded that Japan stop killing Chinese. You could try reading some basic history and you’d learn some stuff.

Are you talking about the embargo because Japan invaded Indochina in 1940? That's quite a while after Japan invaded China. I can remember nothing about the US's actions being linked to Chinese deaths, and google throws nothing up. and the US Department of State's Archive https://2001-2009.state.gov/r/pa/ho/time/wwii/88734.htm doesn't back up your claim either.

Can you link to your claim?

Are you talking about the embargo because Japan invaded Indochina in 1940?

1. Japan didn't invade Indochina in 1940.
2. The embargo was for the invasion of China, not for Indochina (which came after the Pearl Harbor attack in 1941).

Try reading a history book.

Edit. Ahh, you probably mean French Indochina (i.e. Vietnam), not the Indochina region. The U.S. embargo was the result of Japan's invasion of China, not Vietnam. The economic embargo started long before the Japanese invasion of Vietnam. After Pearl Harbor, Japan invaded the Indochina region. The embargo was because of China, not Indochina.

During World War II, on September 22, 1940, Japan invaded Vietnam and began constructing military bases to strike against the Allies in Southeast Asia. Japanese troops remained in Vietnam until their surrender to the Allies in 1945.

In the 1930s, Japan expanded slowly into China, which led to the Second Sino-Japanese War in 1937. In 1940, Japan invaded French Indochina in an effort to embargo all imports into China, including war supplies that were purchased from the US. That move prompted the US to embargo all oil exports, which led the Imperial Japanese Navy (IJN) to estimate it had less than two years of bunker oil remaining and to support the existing plans to seize oil resources in the Dutch East Indies.

Japan expanded slowly into China, which led to the Second Sino-Japanese War in 1937

Which is when the U.S. began the economic embargo against Japan. It didn't become a total embargo until 1941, but it was the invasion of China that started the ball rolling.

and why, having dropped the first there was any need to drop the second.

The answer there is one is Uranium and the other plutonium. Which works best on people and infrastructure. If they weren't interested in the results(as someone here is suggesting), both bombs would have been made of the same stuff.

both bombs would have been made of the same stuff.

They'd already realised that plutonium was "better" for bombs. Gadget, the bomb in New Mexico was a plutonium bomb. the Hiroshima bomb was a "gun type" (fires something into something else) and research had started earlier on that type, then they realised that imploded cores were "better" and stopped development in favour of the Mk3 (Fat Man) There was only ever going to be one Little Boy bomb. If they'd have waited, both would've been plutonium imploded core types.

The answer there is one is Uranium and the other plutonium. Which works best on people and infrastructure. If they weren’t interested in the results(as someone here is suggesting), both bombs would have been made of the same stuff.

That was explained above. Uranium bombs are much simpler to make, but implosion bombs can be made much more powerful. The U.S. made both because they weren't sure if the plutonium bomb would succeed. The uranium bomb was a fall back. Bombs of the same explosive yield give the same results, apart from how much fallout is produced. The atomic bomb attacks weren't tests of the effects of atomic bombs, they were intended to force Japan to surrender. If the U.S. wanted to compare the bombs' performance, they would have done it under controlled conditions, not an operational mission.

I can remember nothing about the US’s actions being linked to Chinese deaths

USS Panay Incident was 1937 wasn't it? Most folks now think that the Japanese deliberately targeted it and the US Oil tankers. Bombed by the Japanese while it was rescuing Chinese civilians in Nanking.