Question: The first Battle of Reading took place following an invasion of Wessex by the Danes, led by which Viking leader?
Context: In the First Battle of Polotsk, which took place on 17–18 August 1812, Russian troops under the command of Peter Wittgenstein fought French and Bavarian troops led by Nicolas Oudinot near the city of Polotsk, halting Oudinot's advance toward Saint Petersburg.  The First Battle of Polotsk should be distinguished from the Second Battle of Polotsk which took place during the same campaign two months later. The Battle of Reading took place on 9 December 1688 in Reading, Berkshire.  It was the only substantial military action in England during the Glorious Revolution and ended in a decisive victory for forces loyal to William of Orange.  It was celebrated in Reading for hundreds of years afterwards. The Battle of Basing was a battle on 22 January 871 at Old Basing in what is now the English county of Hampshire.  It was one of a series of battles that took place following an invasion of the then kingdom of Wessex by an army of Danes.  Both battle and campaign are described in the "Anglo-Saxon Chronicle". The First Viking Age in Ireland began in 795, when Vikings began carrying out hit-and-run raids on Gaelic Irish coastal settlements.  Over the following decades the raiding parties became bigger and better organized; inland settlements were targeted as well as coastal ones; and the raiders built naval encampments known as longphorts to allow them to remain in Ireland throughout the winter.  In the mid 9th century, Viking leader Turgeis or "Thorgest" founded a stronghold at Dublin, plundered Leinster and Meath, and raided other parts of Ireland.  He was killed by the High King, Máel Sechnaill mac Máele Ruanaid, which was followed by several Irish victories against the Vikings and the seizure of Dublin in 849.  Shortly after, a new group of Vikings known as the Dubgaill ("dark foreigners") came to Ireland and clashed with the earlier Viking settlers, now called the Finngaill ("fair foreigners").  The wavering fortunes of these three groups and their shifting alliances, together with the shortcomings of contemporary records and the inaccuracy of later accounts, make this period one of the most complicated and least understood in the fledgling city's history.  In 853 a Viking warlord called Amlaíb (, possibly Olaf the White) arrived and made himself king of Dublin.  He ruled along with his brothers Ímar ("Ívarr", possibly Ivar the Boneless) and Auisle ("Ásl").  For the next fifteen years or so, they used Dublin as their base for a series of campaigns against Irish kingdoms.  During these conflicts they briefly allied themselves with several Irish kings.  The Dublin Vikings also carried out a number of raids in Britain at this time.  The deaths of Ivar (c.873) and Olaf (c.874) were followed by internecine conflict among the Vikings.  Although intermittent warfare between the Vikings and the Irish continued, these inner conflicts weakened the Viking colonies and made it easier for the Irish to unite against them.  In 902, Cerball mac Muirecáin, king of Leinster, and Máel Findia mac Flannacáin, king of Brega, launched a two-pronged attack on Dublin and drove the Vikings from the city.  However, in 914 the Vikings now known as the Uí Ímair (House of Ivar) would return to Ireland, marking the beginning of the Second Viking Age. The First Transjordan attack on Amman (known to the British as the First Attack on Amman) and to their enemy as the First Battle of the Jordan took place between 21 March and 2 April 1918, as a consequence of the successful Battle of Tell 'Asur which occurred after the Capture of Jericho in February and the Occupation of the Jordan Valley began, during the Sinai and Palestine Campaign of World War I.  During the First Transjordan attack large incursions into Ottoman territory occurred.  Firstly the Passage of the Jordan River, was successfully captured between 21 and 23 March, followed by the first occupation of Es Salt in the hills of Moab between 24 and 25 March.  The First Battle of Amman took place between 27 and 31 March when the Anzac Mounted Division and the Imperial Camel Corps Brigade (fighting dismounted as infantry) were reinforced by two battalions of 181st Brigade followed by a second two battalions from the 180th Brigade (60th London Division) and artillery.  The Fourth Army headquarters located in Amman was strongly garrisoned and during the battle received reinforcements on the Hejaz railway, the strength of which eventually forced the attacking force to retire back to the Jordan Valley between 31 March and 2 April.  The Jordan Valley would continue to be occupied by the Egyptian Expeditionary Force (EEF) through the summer until the middle of September 1918 when the Battle of Megiddo began. Bagsecg (also known as Bægsecg or Bagsec) (died 8 January 871) was a Viking leader commonly referred to as a king.  In 870 or 871 he led The Great Summer Army to England.  His forces joined those of the Great Danish Army which had already overrun much of England.  He and Halfdan Ragnarsson became the leaders of a joint invasion of the Kingdom of Wessex.  He was killed at the Battle of Ashdown, fighting a West Saxon Army led by King Ethelred and his younger brother, the future Alfred the Great. The Battle of Englefield was a battle on 31 December 870 at Englefield, near Reading in what is now the English county of Berkshire.  It was one of a series of battles, with honours to both sides, that took place following an invasion of the then kingdom of Wessex by an army of Danes, during which the Danes had established a camp at Reading.  Both battle and campaign are described in the "Anglo-Saxon Chronicle". The Battle of Williams Station was a minor skirmish during the Pyramid Lake War of 1860.  The fight took place following the defeat of Major William Ormsby at the First Battle of Pyramid Lake as U.S. Volunteers entered the war. The first Battle of Reading was a battle on 4 January 871 at Reading in what is now the English county of Berkshire.  It was one of a series of battles, with honours to both sides, that took place following an invasion of the then kingdom of Wessex by an army of Danes led by Bagsecg and Halfdan Ragnarsson in an attempt to conquer Wessex.  Both battle and campaign are described in the "Anglo-Saxon Chronicle", and this account provides the earliest known written record of the existence of the town of Reading. The Battle of Marton or Meretum took place on approximately 22 March 871 at a place recorded as Marton, perhaps in Wiltshire or Dorset.  According to the Anglo-Saxon Chronicle, the battle took place "about two months" after a prior loss to the Danes at the Battle of Basing, in which both sides suffered heavy losses.  King Æthelred of Wessex and Prince Alfred fought against the Danes, who were possibly under the leadership of King Halfdan Ragnarsson, who had commanded many of the earlier battles that year.  The Chronicle tells us that the forces were split in two and that the Danes were initially put to flight, but at the end of the day and after "much slaughter" the Danes remained masters of the field.  Among the fallen was Bishop Heahmund.
Answer: Bagsecg