Earlier quoted context omitted.
Leader of the world and leader of the free world are two different things. As far as I know, no country in the past century has called itself either of them. I would not be surprised if the title was applied to countries such as the Imperium Romanum and 中國 in the distant past. Even more distant would be possibly Greece given that it’s leader was called the King of Kings around the time of Troy. However, I believe bot…
With all due respect, you're being pedantic. No, we're not talking about official titles bestowed from on high. The US has been considered the world leader from WW2 until sometime in the 21st century, partly because nobody else wanted to claim it, partly because American foreign policy is aggressive, partly because the American economy was the most robust on in the aftermath of WW2.
Saying that the U.S. got its leadership position because no one else wanted it is historically incorrect. Following WWII, every other major democratic power was in ruins while the U.S. was the sole major power left in tact. Without any military attacks on U.S. soil, U.S. military strength had skyrocketed during the war. As the war progressed, the U.S. attained the status of a great power and by the war’s end had become a nascent super power. U.S. strength continued to grow after the war due to the threat of the Soviet Union. Meanwhile, the other great powers never fully recovered militarily since they focused on their economies while relying on the U.S. for security. The U.S. gained its leadership position because no one else existed that could claim it.