Why did Britain arm Japan pre-ww1 to 1920s?

Yet the USA didn't have a Treaty Port like all the others, since 1864.

The US wanted Trade, sure, but was doing things differently than the other Great Powers. US Merged their holdings at Shanghai with the UK to form the International in 1864, and released options in Tientsin in 1880, but retained the so called Treaty Rights for how US Citizens were to be treated across the Chine territory.
While the UK had the most Trade of all the Treaty Ports by 1900(over 70%) US was at 8%, there were 1,400 British missionaries and 1,000 were American. The next largers groups were the Swedes and Norwegians, around 100.
Doing things differently was the whole point.
 
The British did not "arm Japan". Japan had the money, especially after defeating China, to buy warships from foreign yards. Britain was the obvious choice as the leading naval power.
 
A drop in the bucket in the greater scheme of things. It helps but GB wouldn't have sold them unless it thought it was in its interest. Containing Russia in the east was was more important than the money.
The monetary benefit from selling weapons isn't really in money earned. More in supporting your own armaments industry so it can afford more capital items and people are more skilled.

It's valuable to note that in the Anglo German naval Arms race before ww1 British ships were generally cheaper to order and quicker to build than German ships. A big part of that is that the shipyards and the rest of the naval industrial complex was more developed and a big part of that was foreign orders.
 
A drop in the bucket in the greater scheme of things. It helps but GB wouldn't have sold them unless it thought it was in its interest. Containing Russia in the east was was more important than the money.

There was more than just arming them - Britain cashed in on Japans effort to drag itself kicking and screaming into the Industrial age.

1 interesting side note of Britains 'time' in Japan was a British Engineer called Edward William Barton-Wright who was one of the principle individuals that introduced Eastern Martial arts to the Western world in the form of Barttisu (a combination of elements of boxing, jujitsu, cane fighting, and French kickboxing) and you have all seen it in action as Sir Arthur Conan Doyle's "Sherlock Holmes" is proficient in the art

But granted there was truth in having the desire for a friendly nation in that part of the world
 
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