LONDON — Imagine if Bosnia’s national soccer team had not been able to take its wunderkind forward Edin Dzeko to the World Cup in Brazil last summer. Or if Ivory Coast had not been able to take stalwarts like Didier Drogba or Yaya Touré. Or if United States Coach Jurgen Klinsmann had not been allowed to put Tim Howard, perhaps the most famous American playing abroad, on his roster.

In rugby, situations like these are a reality. While world championship tournaments are generally thought to be the pinnacles of their sports — the kinds of events that include all the top stars — the Rugby World Cup is different. Strange as it may seem, one of the more interesting aspects of the tournament, which began in England on Sept. 18 and will conclude on Oct. 31, is that some of the best players are not playing.

The problem mostly affects the pre-eminent players from the so-called Tier 2 countries, where the domestic rugby leagues are of middling (or nonexistent) quality. When stars from countries like Samoa, Tonga or Uruguay go to Europe to play in the best leagues, the clubs there happily sign them to contracts that may imply — or overtly state — that they cannot also play for their national teams.