Following the war he was accused of taking part in Gerard's conspiracy to assassinate Oliver Cromwell and pleaded guilty to a charge of high treason, one of only two men in English history to do so.

[3] During the English Civil War he rose to the post of colonel in the Royalist army, having previously seen military service on the Continent; he served under Prince Rupert in the garrison of Bristol.

[2] In 1646 he was involved in surrendering the garrison of Ludlow to Parliamentary forces under John Birch, following which he went into exile.

[2] In May 1654 Fox, Peter Vowell, and Fox's cousin John Gerard were arrested for plotting to attack Lord Protector Oliver Cromwell's bodyguard with a party of cavalry, and to assassinate him as he travelled to Hampton Court.

[4] At their trial by the High Court of Justice sitting in Westminster Hall on 30 June 1654, Fox pleaded guilty.