All the fighting of the thirty odd years which established the Danish dynasty in England and led up to the great reign of Canute, has for its pivot or centre the estuary of the Thames and in successive attacks upon the defence of, or in the alliance of London. Ethelred the Unready, when he forms his first plans against Olaf, gathers his fleet in the Pool of London--though he does nothing with that fleet (992). When, two years later (994), Olaf makes his alliance with Sweyn, it is again up the estuary of the Thames that he sails to make his great attack upon the City--but London beat him off. Though London is attacked again from the _land_ in 1009 and with very great vigour in 1013, Edmund Ironside, a boy who all but saved his house, fought alternately north and south of the estuary against the armies landed from the invading fleet. That campaign (the campaign of 1016) is as excellent an example of the part the Lower Thames played in the warfare of this island before the Norman Conquest as one could choose. In the month of May Canute makes his attempt to reduce the capital. He cut a canal through the alluvium of Suffolk to get his ships round the Bridge head, so that he held London from the River side both above bridge and below, and he dug his trench all round the north, east, and west over against the walls. But he did not reduce the town. Edmund Ironside coming up from the west, re-entered it. The invader retired to an entrenched camp at Greenwich. When later the whole invading fleet had dropped down the Thames and sailed out of the estuary, you get a strategical playing north and south of that obstacle which is most illuminating. Canute attacks the Suffolk shore. Edmund marches thither, having the estuary to the south of him. Canute then sails across the mouth of the Thames and attacks Kent shore. Edmund counter-marches, _is compelled to take the long route crossing the Thames at London_, and finds the Danish advance at Otford to the south of the estuary. He defeats it. Canute thereupon once more crosses the mouth of the Thames and attacks the Essex shore, and once more Edmund goes back by the long land route, crosses at London, gets to the north of the estuary, and fights and loses (by treachery) his great action on the Crouch at Ashington, near Rochford. You could not have within a shorter space of time a clearer view both of what the Lower Thames meant as an avenue of approach to invasion during the Dark Ages that were coming to a close, and what it was to mean as an obstacle to a passage of armies during the centuries of the Middle Ages which were about to open. The Lower Thames played no great part between this date, 1016, and the invasion of William the Conqueror. A fleet seems to have stood on it perpetually for the defence of the kingdom under Canute, but did not serve as an avenue of invasion, nor was that fleet brought to action. [Illustration: PANGBOURNE]