During the 1970s in particular, rival groups of West Ham fans from neighbouring areas often did battle with each other at games, most often groups from the neighbouring districts of Barking and Dagenham. The Inter City Firm were one of the first "casuals", so called because they avoided police supervision by not wearing football-related clothing and travelled to away matches on regular InterCity trains, rather than on the cheap and more tightly policed "football special" charter trains. The group were an infamous West Ham-aligned gang. As the firm's moniker "inter city" suggests violent activities were not confined to local derbies – the hooligans were content to cause trouble at any game, though nearby teams often bore the brunt. Both the 1989 film The Firm (starring Gary Oldman), and the 2005 film Green Street (starring Elijah Wood and Charlie Hunnam) are based upon West Ham hooligan firms. Rivalries West Ham have strong rivalries with several other clubs. Most of these are with other London clubs, especially with Tottenham Hotspur in an East versus North London derby and with Chelsea in an East versus West London rivalry. The rivalry between West Ham and Tottenham has been fuelled by players such as Michael Carrick, Martin Peters, Paul Allen, Jermain Defoe and Scott Parker leaving the Hammers to join Tottenham. The rivalry deepened with the appointment of former Hammers manager Harry Redknapp as Tottenham's manager. Since the 2006–07 Premier League season, West Ham have developed a strong rivalry with Yorkshire club Sheffield United due to the dubious circumstances surrounding the transfer of Carlos Tevez, who helped West Ham avoid relegation at Sheffield United's expense. The oldest and fiercest rivalry is with Millwall. The two sides are local rivals, having both been founded by employees of local companies, with players living in the same localities. The early history of both clubs are intertwined, with West Ham proving to be the more successful in a number of meetings between the two teams at the time, resulting in West Ham being promoted at the expense of Millwall. Millwall later declined to join the fledgling Football League while West Ham went on to the top division and an FA Cup final. Later in the 1920s, the rivalry was intensified during strike action which Isle of Dogs-based companies (i.e., Millwall fans) refused to support, breeding ill will between the two camps, the bitterness of this betrayal enduring for years.