h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This tactic is a national UK police standard adopted for appraoching a building where the occupants are hostile to their entry.

It is usually called a six man entry team: They form two rows of three, the front holding their shields to the front. the rear holding their shields over their heads, sloping forwards so that any fridges thrown slide off.
When they reach the building the 'roof'shields are placed up over the door to provide a safe place to effect an entry through the door. Once the entrance has been breached then the whole team enters.

Variations are used for going through doors where there is an 'angry man' opposing entry, and also for negotiating up staiircases against hostiles.And also for making progress along roads during riots. Although the current thinking is to be much more flexible with rapid movement. The big six foot shields used in the Brixton riots have been replaced with shorter shields and round shields for faster deployment.

Old tricks stay a long time and remain effective

it is a shame to think that we are such a violent society that we need to adopt such methods. But i agree that because we DO live in a society like this it is a good method to use and i guess a very succesfull one.

The one thing that defeats a shield-wall of any kind is a well-aimed petrol bomb. Blazing liquid finding its way through the gaps, or sripping down from above, can reduce an otherwise well-presented and disciplined assembly of police or Toms with riot shields to some dissarray. (Evidence: Northern Ireland)

Surely the Romans would have come across Greek Fire at some point (ie, blazing liquid thrown in bomb form and used from classical times to the middel ages) with similar results?