Sixth Amendment: to prevail under Strickland, Porter must show that his counsel’s deficient performance prejudiced him. To establish deficiency, Porter must show his “counsel’s representation fell below an objective standard of reasonableness.” 466 U. S., at 688. To establish prejudice, he “must show that there is a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different.” Id., at 694. Finally, Porter is entitled to relief only if the state court’s rejection of his claim of ineffective assistance of counsel was “contrary to, or involved an unreasonable application of” Strickland, or it rested “on an unreasonable determination of the facts in light of the evidence presented in the State court proceeding.” 28 U. S. C. §2254(d). Because the state court did not decide whether Porter’s counsel was deficient, we review this element of Porter’s Strickland claim de novo. Rompilla v. Beard, 545 U. S. 374, 390 (2005) (…) counsel had an “obligation to conduct a thorough investigation of the defendant’s background.” Williams v. Taylor, 529 U. S. 362, 396 (2000); under Strickland, a defendant is prejudiced by his counsel’s deficient performance if “there is a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different.” 466 U. S., at 694; Porter must show that but for his counsel’s deficiency, there is a reasonable probability he would have received a different sentence. To assess that probability, we consider “the totality of the available mitigation evidence—both that adduced at trial, and the evidence adduced in the habeas proceeding”—and “reweigh it against the evidence in aggravation.” Williams, supra, at 397–398; see Penry v. Lynaugh, 492 U. S. 302, 219 (1989) (“‘evidence about the defendant’s background and character is relevant because of the belief, long held by this society, that defendants who commit criminal acts that are attributable to a disadvantaged background . . . may be less culpable’”); under Florida law, mental health evidence that does not rise to the level of establishing a statutory mitigating circumstance may nonetheless be considered by the sentencing judge and jury as mitigating. See, e.g., Hoskins v. State, 965 So. 2d 1, 17–18 (Fla. 2007) (per curiam). Indeed, the Constitution requires that “the sentencer in capital cases must be permitted to consider any relevant mitigating factor.” Eddings v. Oklahoma, 455 U. S. 104, 112 (1982); it is unreasonable to discount to irrelevance the evidence of Porter’s abusive childhood, especially when that kind of history may have particular salience for a jury evaluating Porter’s behavior in his relationship with Williams. It is also unreasonable to conclude that Porter’s military service would be reduced to “inconsequential proportions,” 788 So. 2d, at 925, simply because the jury would also have learned that Porter went AWOL on more than one occasion. Our Nation has a long tradition of according leniency to veterans in recognition of their service, especially for those who fought on the front lines as Porter did. Moreover, the relevance of Porter’s extensive combat experience is not only that he served honorably under extreme hardship and gruesome conditions, but also that the jury might find mitigating the intense stress and mental and emotional toll that combat took on Porter; (fn. 9) cf. Cal. Penal Code Ann. §1170.9(a) (West Supp. 2009) (providing a special hearing for a person convicted of a crime “who alleges that he or she committed the offense as a result of post-traumatic stress disorder substance abuse, or psychological problems stemming from service in a combat theater in the United States military”); Minn. Stat. §609.115,Subd. 10 (2008) (providing for a special process at sentencing if the defendant is a veteran and has been diagnosed as having a mental illness by a qualified psychiatrist); we do not require a defendant to show “that counsel’s deficient conduct more likely than not altered the outcome” of his penalty proceeding, but rather that he establish “a probability sufficient to undermine confidence in that outcome.” Strickland, 466 U. S., at 693–694. This Porter has done (U.S.S.Ct., 30.11.09, Porter v. McCollum, Per Curiam).