He fought in
The Battle of Naseby on 14 June 1645 alongside his brother Colonel
Thomas Rainsborough. In the New Model Army, he was a Captain serving for Colonel Thomas Sheffield as seen in the
Naseby order of battle. However, his radicalism emerged early. In May 1647 in
Saffron Walden Essex at a meeting with Parliamentary commissioners, he testified against Colonel Sheffield. Rainsborowe stated that the army was demoralised. He then outlined the complaints of his men. The army did not support their Colonel and Sheffield was afterwards removed. Rainsborowe was then promoted to the rank of Major under Colonel
Thomas Harrison. The Saffron Walden Debates of May 1647 are considered the prelude to the Putney Debates of October 1647. In these debates the army wanted certain rights and freedoms. These debates and a
Second English Civil War (1648–1649) would lead to the
Commonwealth of England (1649–60) when England became a Republic. Rainsborowe showed his Leveller convictions during the army's
Putney Debates at the end of October. William's brother, Thomas Rainborowe, was more fiery in his Leveller speeches, and he was murdered by
Royalists in a bungled kidnap attempt in
Doncaster in October 1648. William led the funeral, and a pamphlet called
The Second Part of Englands New Chaines Discovered of that year discussed Rainsborowe's attempts at finding justice and the resistance of the upper classes. William Rainsborowe's cornet, according to the
Dictionary of National Biography, was, during this time, a depiction of the severed head of
Charles I and the motto
salus populi: suprema lex ("let the good of the people be the supreme law"). ==Radicalism and controversy==