Charles I's reign (1625-1649) was plagued by conspiracy theories, which could likely be considered a contributing factor to his downfall during the English Civil War. Generally, these conspiracy theories claimed that the King was a covert Catholic/Catholic-sympathizer, and that he intended to bring in foreign troops in order to suppress religious and political liberties in England.
Early in Charles' reign (1625-1635), this fear of the King colluding with foreigners was promoted by some of the English Puritans. Two factors caused this theory: The first was that the King had married a foreign (French) Catholic bride. The second factor was that the royal court was promoting Arminianism, an alternative brand of Protestantism which was opposed by the Puritans on the grounds that Arminianism was too close to Catholicism.
Conspiracy theories became more mainstream as the English Civil War drew closer and tensions began to rise in England. By the early 1640s, the Earl of Strafford was conspiring to have the King bring in troops from Ireland in order to suppress dissent within England. Furthermore, King Charles (who, I should mention, was himself a Scot) traveled to Scotland in the summer of 1641 in the hopes of gaining Scottish military support. Once made aware of these secret plots, Parliament entered a state of panic and demanded control over the English army.
Members of parliament saw these military plots, and assumed the existence of further plots of a foreign, religious nature that they were still unaware of. John Pym, one the most prominent political opponents of the King in Parliament, publicly warned that Catholicism was a religion which was "destructive to all others," and that the Papists in the royal court, such as the Queen, would "if they have power...bring [England] to ruin." [1]
This rhetoric quickly spread in the months preceding the English Civil War. Examples of this include: Petitioners from the county of Surrey who feared that the "malice of enemies abroad" would result in aggression toward England[2]. A private diary entry by a chap named James White includes fears that there existed "forces preparing in Denmark and France against England."[3] In 1641, a speech was given in the House of Commons about a woman who was considered a danger because she possessed weapons and contact with people in Ireland.[4] By mid 1641, some coastal towns began preparing militarily in anticipation of a foreign naval invasion that would never arrive. One military directive calls for every shire to raise a militia against "foreign princes" who "will be able to land at one time 2 or 3 thousand men." [5]
[1] Fletcher The outbreak of the English Civil War pg 138
[2] Fletcher pg 214
[3] Stoyle From Deliverance to Destruction pg 170
[4] Peacey's chapter in Conspiracies and Conspiracy Theory in Early Modern Europe pg 125
Secondary Sources: Peacey's chapter as listed listed above is a good source on the early Puritan fears of Arminianism. My main source for the conspiracy theories right before and during the Civil War is Stoyle's Soldiers and Strangers.