system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What makes the US dollar an international currency?
gpt
Traditionally, governments aligned their currency to the gold standard. That meant that all these countries had to keep stockpiles of gold securing their economy, so anyone could redeem their currency for gold upon demand.

In 1944 the leaders of may other countries met in Bretton Woods, New Hampshire and agreed to drop the gold standard and instead peg their currency to the value of the US Dollar.  At that time, the dollar was backed by its value in gold, and the United States held the largest gold reserves. This allowed other countries to back their currencies with dollars, rather than gold.

It was this way until the 1970's, when Nixon decoupled the US Dollar from gold values, but by then the USD was already the standard used for foreign transactions, and the USD was very stable. 

The Euro has crept into the market (aprox 31% of international reserves) but primarily in the European block, and China has been making moves to convince other countries to switch to the CNY, but outside of traditionally anti-US countries it has not really caught on.