Final answer:
Cassius has letters delivered to Brutus in Shakespeare's 'Julius Caesar' to manipulate him into joining the conspiracy against Caesar, by suggesting popular support for Brutus's leadership and opposition to Caesar's autocratic rule.
Step-by-step explanation:
The letters delivered to Brutus by Cassius in Shakespeare's Julius Caesar are part of a manipulative ploy by Cassius to persuade Brutus to join the conspiracy against Julius Caesar. Cassius hopes to sway Brutus against Caesar by fabricating evidence of popular support for Brutus's assumed opposition to Caesar's growing power.
The letters are not for celebration, a warning, a political campaign support request, or a duel challenge, but rather to inflame Brutus's sense of honor and republican virtue to convince him that overthrowing Caesar is justified by the will of the people.