Answer based on context:

Regent Moray and his secretary John Wood tried to gain English support by producing the Casket letters in England, which were intended to incriminate Mary in the death of Lord Darnley. Moray also raised money in London by selling items from the Scottish crown jewels, including what was claimed to be a unicorn horn. Queen Elizabeth and her advisors were at first reluctant to intervene but their actions, and support of Moray, served to prevent reconciliation in Scotland. The Regent Moray was assassinated in January 1570 by a member of the Hamilton family. Elizabeth sent an army into Scotland in May 1570, which reached Glasgow, where the Queen's party were besieging Glasgow Castle. The army was commanded by the Earl of Sussex from Berwick, its leader in Scotland was William Drury, styled "Captain General." The Earl of Lennox was in their company, arriving at Edinburgh on 14 May 1570. The Marian lords abandoned their siege of Glasgow Castle before the English arrived on 18 May, and returned to their homelands, the Hamiltons to Arran and Craignethan Castle, and Drury attempted a siege of Dumbarton Castle. For Elizabeth's foreign policy this intervention had the effect of making France and Spain less likely to offer tangible pro-Marian support. After the Rising of the North and the discovery of the Ridolfi plot which further damaged Mary's reputation, Regent Mar and James Douglas, Earl of Morton were able to broker the deployment of an English army against Mary's supporters at Edinburgh Castle. The English diplomat Henry Killigrew worked on the reconciliation of the Scottish nobility at Perth in February 1572, where many promised not to support Mary as Queen. The fall of Edinburgh Castle concluded the civil war.

Which event occurred first, when Elizabeth sent an army into Scotland where the Queen's party were besieging Glasgow Castle, or when Regent Moray was assassinated?
when Regent Moray was assassinated