Depending on who you ask, Major League Soccer is only days away from a work stoppage that could either postpone the 2015 season for a few weeks or end professional soccer in the U.S. America’s top professional soccer league has a collective bargaining agreement that expired last month, and if the league and the Major League Soccer Players Union (MLSPU) can't reach an agreement by the March 5th opening day, players are prepared to strike.

At issue is the single most contentious concept in American sports labor history: free agency. Major League Soccer operates as a single entity, and rather than signing with an individual team, its players sign contracts with the league as a whole. This policy has resulted in American players like Herculez Gomez frozen out of MLS after achieving success in foreign leagues. United States Men's National Team (USMNT) midfielder Jermaine Jones's city of residence was left up to a random draw. Through this system, players remain attached to the teams they entered the league with until the team no longer has use for them—if they'd rather play for Seattle than Portland or Chicago than Washington, D.C., too bad. MLS contends the players can just join an international team if they don't like it.

Jones is one of the few MLS players for whom international play is a realistic option. Fellow USMNT teammate Michael Bradley is another: Bradley has played in both Italy and England and is under contract to earn $6.5 million for Toronto FC in 2015. He was was one of 15 players to earn seven figures in MLS in 2014, a special class of players largely including fellow top American players like Clint Dempsey ($6.7 million) or fading international stars like Thierry Henry ($4.35 million) and Robbie Keane ($4.5 million).

The majority of MLS athletes play for salaries far closer to the league minimum of $36,500. Despite the presence of highly paid stars like Bradley, Dempsey and Jones, MLS's median salary last year was under $100,000. For them, the international market does not exist. The costs associated with moving and living internationally are high. If a player finds himself cut early on, or if a deal falls through at the last minute—as happened to former New England Revolution center back A.J. Soares this winter—players can be left scrambling. The mid-level American player has zero leverage for negotiation.

Major League Baseball employed similar tactics in the days of the reserve clause, a rule that kept professional baseball players tied to their parent clubs in perpetuity. In 1945, as a number of top players were returning from service in World War II, the Mexican League offered a number of major leaguers as much as 10 times their American salaries to play south of the border. Baseball commissioner Albert B. "Happy" Chandler responded by suspending for five yearsany player who jumped to the Mexican League. Danny Gardella, a Yankee in 1944 and 1945, challenged the suspension and the casemade it all the way to the Supreme Court. Gardella won a $60,000 settlement and the players who jumped to the Mexican League eventually received amnesty from MLB, but the threat of suspension was enough to keep major leaguers from striking at the status quo.