unable to get reinforcements or outside aid, the London elements of the army would demand to go home, and the one remaining significant Parliamentarian force would find itself pinned down, while Charles, with secure bases at Oxford and Bristol, would be able to starve them into surrender while other Royalist armies ran rampant through Britain. Because of this, Essex had no choice but to try to return to London. Travelling back across the Cotswolds, as he had done to reach Gloucester initially, would expose the Parliamentarians to Charles's cavalry on open ground. The first alternative was to march southeast to the River Kennet and cross it, going through Newbury and returning to Reading's fortifications, thus evading the Royalists and allowing for a safe retreat to London. The disadvantage to this was the time it would take to cross the relatively open land between Essex's position and the Kennet. The second, and the option Essex initially took, was to go north, either to make battle in more advantageous circumstances or to evade the Royalists. If Essex was able to cross to the west bank of the River Avon, he could secure the bridges across it and prevent the Royalists from crossing it and confronting his army. His cavalry advanced to Upton to screen the main force from Royalist interference on 11 September, with the rest of the soldiers quickly following. The Royalists were left wrong-footed; Charles did not discover Essex's retreat for another 24 hours, during which the gap between the armies widened. The Royalists finally began to march on 16 September, with Rupert's cavalry streaming ahead to try to disrupt the Parliamentarian retreat. By 18 September, Rupert's force had caught up to the Parliamentarians outside Aldbourne, Wiltshire. Essex had lost his advantage; Parliamentarian intelligence reports had convinced him that Charles was heading towards Oxford and had given up the campaign. In fact Charles was barely away, but the complacency such reports induced meant that one contemporary source stated the Parliamentarians were marching barely a day, allowing the Royalists to quickly catch up. Suitably chastened by the discovery of his error, Essex increased the pace of his retreat, with the Royalists pursuing closely. Both sides were heading for Newbury, on roughly parallel routes; the Royalists' route took them through Faringdon and Wantage, increasing the distance they would have to travel to while the Parliamentarians had to travel only 20. Charles reacted