The siege of Oxford took place during the Anarchy, a period of civil war following the death of Henry I of England without a male heir, in 1142. Fought between his nephew, Stephen of Blois, and his daughter, the Empress Matilda (or Maud), who had recently been expelled from her base in Westminster and chosen the City of Oxford as her new headquarters. Oxford by now was effectively a regional capital and important in its own right. It was a well-defended city with both rivers and walls protecting it, and was also strategically important as it was at a crossroads between the north, south-east and west of England, and also not far from London.

By now the civil war was at its height, yet neither party was able to get an edge on the other: both had suffered swings of fate in the last few years which had alternately put them ahead, and then behind, their rival. Stephen, for instance, had been captured by Matilda's army in 1141, but later in the year, Matilda's half-brother and chief military commander, Robert, Earl of Gloucester was captured by Stephen's army. Likewise, Matilda had been recognised as "Lady of the English" but had not long afterwards been run out of London.

Stephen believed that all it would take to win the war decisively would be to capture Matilda herself; her escape to Oxford seemed to present him with such an opportunity. Having raised a large army in the north, he returned south and attacked Wareham in Dorset; this port town was important to Matilda's Angevin party as it provided one of the few direct links to the continent that they controlled. He attacked and captured more towns as he returned to the Thames Valley, and soon the only significant base Matilda had outside of the south-west—apart from Oxford itself—was at Wallingford Castle, held by her close supporter Brian Fitz Count.

Stephen's army approached Oxford on 26 September 1142, and according to contemporary accounts, swam his army across the rivers and waterways that blocked the approach to the city. Matilda's small force was taken by surprise. Those that were not killed or captured retreated into the castle; Stephen now controlled the city, which protected him from counterattack. The king knew he was unlikely to be able to take the castle by force—although that did not stop him from using the latest siege technology. He also knew that it would be a long, hard wait before Matilda was starved out. But after nearly three months of siege, conditions for the garrison were dire, and they formed a plan to help the Empress escape from under Stephen's nose. One early December evening Matilda crept out of a postern door in the wall—or, more romantically, possibly shinned down on a rope out of St George's Tower—dressed in white as camouflage against the snow and passed without capture through Stephen's lines. She escaped to Abingdon Abbey and then to Wallingford Castle where she was safe; Oxford Castle surrendered to Stephen the following day, and the war continued punctuated by a series of sieges for the next 11 years.

Background Henry I died without a male heir in 1135, leading to a succession crisis. His only legitimate son and heir, William Adelin, had died in the sinking of the White Ship in 1120. Henry wished his daughter the Empress Matilda, to succeed him, but female succession rights were ill-defined at this time—indeed, there had not been an uncontested succession to the Anglo-Norman patrimony during the previous sixty years. On Henry's death in 1135, his nephew Stephen of Blois claimed and seized the English throne; fighting broke out within a few years, eventually becoming a fully-fledged rebellion against Stephen, as Matilda also claimed the English throne. By 1138 the dispute had escalated into a civil war known as the Anarchy. The Empress Matilda had recently been expelled from Westminster Palace by rebellious Londoners, who had "swarmed out like angry wasps" from London, while Stephen's queen—also named Matilda—approached Southwark from Kent. The Empress Matilda—"in great state", reported James Dixon Mackenzie—evacuated to Oxford in 1141, making it her headquarters and setting up her Mint. Prior to her eviction from Westminster, she had made some political gains, having captured King Stephen and been recognised as "the Lady of the English". Although Matilda never matched the King in wealth, both sides' armies probably ranged in size from 5,000 to 7,000 men.

Oxford Oxford itself had become increasingly important by this period, and, in the words of historian Edmund King, it was "in the course of becoming a regional capital". It also had a royal castle. Its value for whoever held it was not merely symbolic; it was also of great practical value. It was particularly secure, surrounded as it was, says the author of the Gesta Stephani, by "very deep water that washes it all around" and ditches. The Berkshire-Oxfordshire interface area was a contentious one throughout the war, and Oxford particularly was of great strategic value. It was situated at the nexus of the main routes from London to the south-west and from Southampton to the north. Whoever controlled the Oxford area effectively controlled access to London and the north, and for Stephen it provided a bridgehead for attacking Matilda's south-western heartlands.Although the size of the army Matilda took with her to Oxford is unknown, it contained only a few barons with whom she could keep a "small court", and for whom she could provide from the local lands of the royal demesne. Oxford's relative proximity to the capital, suggests Bradbury, also made it a "brave move" on her part; it probably also indicates that she did not wish to move too far and that she intended to return to, and reclaim, London in due course. Matilda recognised that her lack of resources meant that she could not bring the war to a decisive close at this point, and so she sent her half-brother, Robert, Earl of Gloucester, to her husband, the Count of Anjou, to try to bring him and his large, experienced army in on her side. Matilda and the earl probably assumed that she would be safe in Oxford until he returned. This was a crucial period for Matilda, says King, and Gloucester's absence weakened her force further: he left for Normandy on 24 June to negotiate with Anjou, despite, says Crouch, Matilda's situation being "desperate". However, she considered Oxford to be her "own town", commented the 17th-century antiquarian Samuel Daniel. Stephen had recently been so ill that it was feared, temporarily, that he was dying; this created a degree of popular sympathy for him, which had already welled up following his release from Matilda's captivity the previous November. A. L. Poole described the train of events thus:

Matilda and Gloucester, on the other hand, did not know that he was on the road to recovery; if they had, suggests R. H. C. Davis, they might not have delayed or even cancelled his journey. However, they did not, and Matilda's army was effectively left leaderless. Matilda may have been expecting supporters to make their way to Oxford—"to 'make fine' with her" (i.e. to contract themselves to her cause), suggests Edmund King—"but they were under no compulsion to do so". It is likely, says Professor H. A. Cronne, that by now "the tide had turned and already men were quietly leaving her court". John Appleby, too, has suggested that much of her support had by now decided that, in his words, they had "bet on the wrong horse", particularly as she had failed to put up a stand at Westminster or immediately return in force. Stephen, on the other hand, had recuperated in the north of England; he had a solid base of support there and was able to raise a large army—possibly over 1,000 knights—before returning south.

The siege

Following Stephen's recovery, says the author of the anti-Angevin Gesta, the King acted like a man "awakened as out of sleep". He approached Oxford rapidly from the south-west; although the size of his army is unknown, he had already won a series of small but significant victories, punching a gap into the Angevin-controlled south-west. This won him the port town of Wareham—cutting the Angevins' line of communication with their continental heartlands—and Cirencester, as well as the castles of Rampton and Bampton. The capture of those two castles, in turn, cut Matilda's lines of communication between Oxford and the south-west and opened the Oxford road to Stephen on his return. He probably travelled via Sherborne, Castle Cary, Bath and Malmesbury, all of which were held by his supporters (and conversely, suggests Davis, avoided Salisbury, Marlborough, Devizes and Trowbridge, which were held for the Empress).

Stephen arrived at the river bank looking over to Oxford on the evening of 26 September 1142:: "and the date at which the siege was begun is definitely known from William of Malmesbury, 26 September 1142." the city was unprepared for his arrival. David Crouch comments that the King "had chosen his time well": the city's and castle's previous castellan, Robert d'Oilly had died a fortnight earlier and his successor had yet to be appointed. Thus the only military presence in Oxford was the Empress' armed householdmen, a relatively small force of soldiers. They "bravely or foolishly turned out to dispute his crossing of the river", and, thinking themselves secure, taunted Stephen's army from the safety of the city's ramparts, raining them with arrows shot across the river. While the Queen's army offered battle outside the city, Stephen was intent on besieging the castle without a battle, but this meant taking the city first. Stephen's men had to navigate a series of watercourses, what the Gesta describes as an "old, extremely deep, ford". They successfully crossed—at least one chronicler believed them to have swum at one point—and entered Oxford the same day by a postern gate. The Empress' garrison, both surprised and outnumbered, and probably panicking, beat a hasty retreat up to the castle. Those that were caught were either killed or kept for ransom; the city itself was looted and burned. Matilda was thus stranded in Oxford Castle with an even smaller force than that she had entered the city with.