Millwall were formed in 1885 on the Isel of Dogs, and drew a lot of their support from the island and surrounding areas like Poplar, Bow, Stepney, Wapping and also south of the river in Southwark. These were mainly dockers and shipbuilders.
Back in these early times Millwall believe it or not were the best team outside of the FA, and were nicknamed 'the Lions of the South'. The club remained strickly amateir despite several runs to the FA Cup Semi-Finals and being invited as the first London club to join the FA, Millwall turned it down as the southern league was considered as stong at the time. A team called Woolich Arsenal instead took that place, and the rest is, as they say, history.
Meanwhile so other shipbuilders and ironworkers from around the East End also decided to get a factory team together, and so Thames Ironworks FC was born.
Millwall and Thames Ironworks often met in heated league and East End Cup games, with a healthy rivarly between the two sets of dockers and shipbuilders as is natural.
Eventually Thames Ironworks outgrew their home and moved to where they now play and in order to attract more widespread support adopted the name of West Ham United. Whilst Millwall's star dipped West Ham's couldn't stop rising, with a famous FA Cup Final and a place in Division One the reward. Happening around this time was the depression which greatly affected the Docks and in short-terms there was a dispute over pay etc and the entire docks went on strike. Millwall by then had only just moved across the water to New Cross but many islanders and therefore dockers saw them as 'their' team. When the Millwall ship yard (with mostly Millwall fans as employees) broke the strike there was outrage from the other yards (Mostly West Ham fans). This did cause a lot of tension, but how much we can't tell and a lot can be put down to fuzzy rose tinted glasses etc....
Anyhow, the clubs didn't meet for a long time so this tension eventually died out for the most part, especially with both clubs getting gates of 30-40,000 and the dockers no in the minority of the support. The clubs did then have healthy relations and many friendly and testimonial games were played between the two.
But as the 50s turned into the 60s there was new rivarly between the East End ans southeast London. Gang warfare. The Krays and Richardsons were two rival gangs from opposite sides of the river. The media has built up the Krays but it was really the Richardsons who were far more sinister and nasty, the violence was so bad that East End mobsters nicknamed southeast London 'Indian Country' as they said it was lawless and anything could happen. Of course this has developed into a terrace culture, ie who's better east or southeast London? Better fighters? Better dressed? etc...
Then we have the increase in football violence and the timing on Harry Cripp's testimoninal at the Den. West Ham United were the invited team and this gave the two rivals 'firms' the excuse they needed to settle old scores, ie the docks, who's tougher, who will show, who'll be better dressed bla bla bla...the result was carniage in possibly the most violent testimonial ever seen. Then at the next meetings between the clubs, in the league, it just esculated, sadly with deaths on both sides of the divide. After that there was no going back and the hate has been there ever since. How do you forget the death of one of your fans, it isn't like a bad result (4-1 ) that can be forgotten etc...
Also there's the simlarities between the two clubs. Both have very old London values as their base in the hardcore support, both are very passionate about their clubs, and both always get kicked in the teeth but come back for more.
(This post was made by Moody_Blue_MFC on the forum of www.kumb.com)