What competition was Derby County playing in when they achieved their biggest win?

Derby County Football Club achieved their biggest win in the 1973 FA Cup Final. This was a competition that was first established in 1871 and is the oldest football tournament in the world. It is open to teams from England and Wales, with teams from the top four divisions of the English Football League competing for a place in the tournament. The tournament usually runs from August to May, with the final being played at Wembley Stadium in London.

In 1973, Derby County faced off against Leeds United in the FA Cup Final. Derby County had made it to the final following a series of impressive performances in the earlier rounds of the competition, including a 3 - 0 victory over Manchester United in the semi-final. After a tense match, Derby County eventually emerged victorious with a 1 - 0 win, securing their first and only FA Cup win.

This victory was a huge achievement for Derby County, and it was their biggest win to date. The victory lifted them to a new level of prestige, and it was a momentous occasion for the club. The win also earned them a place in the European Cup Winners' Cup, where they achieved success in 1975 by reaching the semi-final.