Do the United States and Japan allow dual citizenship?

That's not true. The United States defaults to dual citizenship; However, Japan does not recognize or allow citizens to have dual citizenship.

United States:

The Supreme Court declared that dual nationality "has long been recognized by law" and that "one person can have and exercise civil rights and perform obligations in two countries at the same time".

The Immigration and Nationality Law does not provide for dual nationality, but earlier bills tend to reduce the occurrence of dual nationality. Although naturalized citizens are required to swear allegiance to their country of origin, the oath does not force them to give up their nationality.

Although the American government adopts a vague policy towards dual nationality, it fully recognizes and allows the existence of dual nationality.

Japan:

Japanese citizens "may not have dual nationality, or they must give up their original nationality after acquiring Japanese nationality."

However, in a few cases, citizens may have dual nationality:

First, those who obtained dual nationality before 1985, that is, before the promulgation of the Nationality Law;

The second is to apply for dual nationality at birth, such as: one parent is not a Japanese citizen and I apply for the nationality of my country; I was born in a territorial country; Wait a minute. But in this case, you must choose your nationality before the age of 22. If you choose Japanese nationality, you lose your second nationality; Otherwise, you will lose your Japanese nationality.