There appears to have been a well-thought-out overall strategy by the Viking leadership in their C9th campaigns in England - with the names and number of leaders for each expedition rarely known except for the so-called 'Great Army' that invaded England in 886 - 871 and took on its Anglo-Saxon kingdoms one by one. Presumably in earlier cases the armies were smaller due to the useage of the word 'Great' for the 866 expedition alone, but an earlier raid up the River Thames in 851 seems to have had around 350 ships (Anglo-Saxon Chronicle entry for the year 851) so it must have had a few thousand men too.
1. The Vikings often attacked when a realm's king was old and/or ailing, eg when they helped a Cornish revolt against king Egbert of Wessex in 838 (he defeated them but died in 839, probably in his sixties), or a king known as a strong warlord had just died, eg when they attacked Wessex after Egbert died and again when his eldest grandson Aethelbald, co-victor of the battle where the 851 invasion was defeated, had died in 860 and been succeeded by his untried brother Aethelbert. In 866 the Great Army started their seizing and settling of AS kingdoms with the weakest big power, Northumbria, as there was a civil war going on - and defeated the two hastily reuniting rival kings. The more war-ready and stronger Mercia and Wessex were left for later.
2. The Great Army of 866, led by the three sons of the late famous warlord Ragnar Lothbrok (Leather Breeches), landed in the weakest and smallest AS kingdom , East Anglia ,and ordered its outnumbered king (St) Edmund to give them supplies and horses to help them invade Northumbria. He did, and they left to head North and invade Northumbria; he presumably thought that they would be preoccupied or defeated there. They won , attacked larger Mercia in 868-9 and forced a treaty and ransom out of its king, and then returned to take Edmund on and defeat and kill him. They then occupied EA. With all the Eastern kingdoms of England neutralised, only then did they move on to take on the further away Wessex - and occupied a major town on a river that their longships could sail up, ie Reading on the Thames, so that when the West Saxon fyrd arrived they could sit it out in the town, with supplies coming up the river, and dare the Saxons to attack a defended position. The Saxons did and were defeated - and won only when the Vikings moved out of the town to chase them across the hills, at Ashdown. The Vikings has the added advantage that when they sat it out in a riverine town they had already looted the local countryside so the Saxons had to bring their own food and/or force more out of the local peasants; and once the fyrd had expired their term of service they would insist on going home and the Vikings would outnumber the AS kin's bodyguard. Alfred stopped them using this trick by only calling up half the fyrd at one time - so when they were legally allowed to go home the other half would be arriving and the Vikings would still face the same number of men. Later on Alfred also started blocking the local rivers with bridges so the V could not bring in any supplies and were starved out or had to break out.
3. This tactic of using a river base town with walls plus a supply route for their ships was the key to their 865-71 invasion - they firstly took York on the R Ouse (Northumbria), then Nottingham on the R Trent (Mercia), then Reading in Wessex on the R Thames. In the 876 attack on Wessex by Guthrum he headed to an unravaged bit of Wessex, ie Dorset , and took Wareham on another river; then promised the besieging King Alfred to leave his kingdom but broke out by land and hurried over to Exeter on the R Exe, down the coast in Devon. This time his sea fleet, bringing his supplies to Exeter safely by sea, was sunk by a storm off the rocky coast at Swanage and he ran out of food and rally had to leave - temporarily. (The 851 invasion had notably centred on the R Thames and the Kent coastal towns, ie using proximity to their ships; the Vikings were defeated when they headed away from their ships into Surrey.) All these campaigns were carefully thought out strategically - it was not just smash and grab. Medieval warfare could be quite sophisticated - despite there being as far as we know no textbooks. So tactics were presumably passed on by one Viking (and Saxon) warlord by word of mouth to the next generation.