The Scottish Act of Security was a response by the Scottish Parliament to the English Act of Settlement of 1701. Queen Anne's final heir, William, Duke of Gloucester had died in 1700, and both Parliaments needed to find a Protestant successor. The English Parliament had settled on Electress Sophia of Hanover, grand-daughter of King James VI of Scotland, without consulting the Scottish Parliament.
The response of the Scottish Parliament was to pass a bill in 1703, requiring that on the death of Queen Anne without issue, the three Estates of the Parliament were to appoint a Protestant successor from the descendants of the Scottish kings, but not the English successor unless various economic, political and religious conditions were met. The bill was refused Royal Assent by the Queen's Commissioner.
The following year 1704 the bill became an Act after the Scottish Parliament refused to raise taxes and sought to withdraw troops from the Duke of Marlborough's army in the War of the Spanish Succession unless Royal Assent was given.
The English Parliament retaliated with the 1705 Alien Act, threatening to cut trade and free movement between the two countries, unless negotiations opened leading either to the repeal of the Act of Security, or (as in the event happened) to the Act of Union in 1707. The end result was the Union of England and Scotland into Great Britain, approximately one hundred years after the Union of the Crowns.