New Cornish Tertia army

The New Cornish Tertia was the Cornish army raised to defend Cornwall during the English Civil War or "War of Five Nations" as it is sometimes known. Cornwalland Wales were staunch Royalist strongholds whilst South East England was held by Parliament, and the remainders of England were in dispute. At the start of hostilities in 1642. King Charles appointed Sir Richard Grenville, 1st Baronet as "General of the West" with authority to raise an army in Cornwall, consisting of four infantry regiments known as the "New Cornish Tertia". Cornwall's rights and privileges were tied up with the royal Duchy and Stannaries and the Cornish saw the Civil War as a fight between England and Cornwall as much as a conflict between King and Parliament. The English Parliament wanted to reduce royal power. This would include the Cornish Parliament which had powers rivalling those of the Parliament of England.

Parliamentary forces invaded Cornwall three times and burned the Duchy archives (at Luxulyan). Grenville's infantry played an important role in the campaign against the Earl of Essex's army at Lostwithiel during the summer of 1644 and he took command at the siege of Plymouth. In 1645 Cornish Royalist leader Sir Richard Grenville made Launceston his base and he stationed Cornish troops along the River Tamar and issued them with instructions to keep "all foreign troops out of Cornwall". Grenville tried to use "Cornish particularist sentiment" to muster support for the Royalist cause and put a plan to the Prince which would, if implemented, have created a semi-independent Cornwall.