U.S. abbreviation in APA style
You can use abbreviations like U.S., IQ, DNA, cm, laser, etc. that have entries in standard dictionaries without defining them first. Here's a couple of APA blog posts expanding on this:


An Abbreviations FAQ
How Do You Spell IQ? Abbreviations as Words in APA Style


I suggest erring on the side of caution if you think the abbreviation might be misunderstood though. However, with the case of U.S. that shouldn't be a problem.