American travelers carry several identity documents that overlap without being equivalent. Each was created for a different purpose, and the differences matter at exactly the moments when they cannot be fixed.
Identity documents answer different questions
A state-issued license or identification card establishes who you are to authorities within the country, and its issuance is governed by the state that issued it.
A passport is issued by the federal government and establishes nationality as well as identity, which is what allows it to function internationally.
Because they come from different issuers for different purposes, one does not substitute for the other wherever the requirement is specifically about nationality.
Domestic identification requirements have their own standards
Federal standards apply to which state-issued credentials agencies accept for certain purposes, including domestic air travel, and states issue compliant credentials that are marked accordingly.
Passports are generally accepted where those standards apply, which is why some travelers carry one domestically rather than replacing a license.
Implementation dates and accepted document lists have shifted repeatedly, so the operating agency's current published list is the only reliable reference.
The passport card has a narrower scope
A passport card is a wallet-sized federal credential that establishes nationality but is limited in where it can be used to enter the country, generally at land and sea entry points within a defined region.
It is not a substitute for a passport book for international air travel, which is the misunderstanding that most often causes a problem at an airport counter.
It is useful for frequent land border crossers and as a durable federal identity document, which is why both exist rather than one replacing the other.
Validity requirements sit on top of possession
Holding a valid document is not always enough, since many destinations impose their own conditions relating to remaining validity, blank pages or onward travel.
Airlines apply those conditions at check-in because they carry responsibility for passengers refused entry, which is why a document can be refused before you reach any border.
Practical handling on a trip
Carrying originals securely, keeping separate copies stored where they can be retrieved without the originals, and knowing which document each leg of the journey requires covers most of the risk.
Requirements are set by governments and are revised frequently, sometimes with little notice, so verifying against official sources close to departure is worth more than any general summary.