Strickland v. Washington (1984):

“A convicted defendant's claim that counsel's assistance was so defective as to require reversal of a conviction or death sentence has two components. First, the defendant must show that counsel's performance was deficient. This requires showing that counsel made errors so serious that counsel was not functioning as the "counsel" guaranteed the defendant by the Sixth Amendment. Second, the defendant must show that the deficient performance prejudiced the defense. This requires showing that counsel's errors were so serious as to deprive the defendant of a fair trial, a trial whose result is reliable. Unless a defendant makes both showings, it cannot be said that the conviction or death sentence resulted from a breakdown in the adversary process that renders the result unreliable.”

(1) Nature of the Error Prong
Whether the counsel’s performance was deficient under the circumstances with performance measured under the standard of prevailing professional norms and
(2) Prejudice Prong
Whether the lawyer's conduct affected, with reasonable probability, the trial's outcome - this relies upon but-for causation: "but for counsel's unprofessional errors, the results of the proceeding would have been different."