Whether we treat the US as singular or plural, it depends on what we want to say about it. For example, if we’re treating the United States as the country, the singular form is OK:
The United States is a federal republic consisting of 50 states.
On the other hand, if we talk about the government in the United States which consists of a number of people, we may use a plural noun:
The United States choose not to overreact to criticism.
From the 18th century to the much of the 19th century, United States had been written as plural; however, towards the latter half of the 19th century, the singular usage came to the fore. In modern English, both the singular and plural usages are acceptable.
This singular plural difference is also evident from many other nouns in English. When we’re talking about the noun team:
The team gets a standing ovation from the crowd at the end of the game.
Here, the noun team is considered one unified thing. However, if the team consisting of several members who have some issues within the team, then this group of nouns can be treated as singular or plural depending on what it refers to.
Similarly, when we talk about the police, the navy, or the army, we can use singular or plural depending on the context. However, one thing is clear: when we consider them as a collection of individuals, we use a plural noun and when they are treated as a unified entity, they are considered plural.