Danvers fell into debt, and from 1630 to 1640 was apparently struggling with creditors. About 1640 he began an active political career in opposition to the king. He refused to contribute to the expenses of the king's expedition to Scotland in 1639, and was returned to the Short parliament of 1640 by Oxford University. In 1642 he took up arms for the parliament, and was granted a colonel's commission, which he held in command of the Wiltshire foot militia until 1650 but did not play a prominent part in military affairs. He gives an account of the opening incidents of the war in letters written to friends from Chelsea in July and August 1642.