what determines what country you play for in the world cup

In the World Cup, you generally play for the country where you’re a citizen, but FIFA also allows some players to qualify through a parent, grandparent, or, in some cases, long-term residency. Once a player appears in an official competitive match for one national team, they’re usually tied to that team and can’t switch freely.

What usually counts

• Birth in the country.
• Citizenship of the country.
• A parent or grandparent born there.
• Residency for a required period, which FIFA uses to limit “passport shopping”.

Switching teams

A player can sometimes change which national team they represent, but only under strict FIFA rules. The key limit is that they must not have been firmly tied by playing in an official senior competitive match, and the switch must fit FIFA’s eligibility conditions.

Simple example

If a player is born in France but has a Nigerian parent and also has Nigerian nationality, they may be eligible for either country until they play an official competitive match that ties them to one side.

TL;DR

Nationality is the main factor, but family background and residency can also make a player eligible. The big line is this: once you play an official competitive match for one country, switching becomes very limited.