Brutus is against standing armies because he believes they are highly dangerous to public liberty and freedom. He references historical examples such as Julius Caesar's Roman army and Cromwell's army in Britain, showing how standing armies can lead to the overthrow of republican governments and the rise of despotism. Brutus argues that standing armies tend to intimidate legislatures and can evolve into forces that dismiss or control the government itself. He sees standing armies as a threat not only because they can support tyrannical rulers but also because they increase the risk of the military overpowering civilian authorities. For these reasons, Brutus recommends severe limitations on the size and use of standing armies to protect the liberties of the people and preserve the republic.

In summary, Brutus's opposition to standing armies rests on their historical association with the loss of freedom, their potential to corrupt or control government, and their inherent danger to the principles of a free republic.