OPINION
ODOM, Judge.This appeal is from a conviction for aggravated assault upon a peace officer, V.T. C.A. Penal Code Sec. 22.02(a)(2). Punishment was assessed by the court at confinement for two years and a fine of one thousand dollars.
The judgment must be reversed because the trial court erroneously excluded rebuttal evidence offered by the appellant.
A lengthy recitation of the facts is not necessary. This prosecution arose out of two fights between appellant and the complaining witness, Deputy Sheriff Alfredo Menchaca. Both occurred during booking procedures at the Maverick County jail.
There were four witnesses to the outbreak of hostilities. Menchaca and another deputy sheriff testified that appellant without provocation attacked Menchaca. Appellant and his mother testified that Men-chaca without provocation or warning suddenly charged appellant in an attacking manner, and that appellant swung at the officer in self-defense. The record is in similar dispute as to the identity of the aggressor in a second fight occurring a few minutes later.
Appellant’s defense was predicated upon a theory of self-defense codified in the new Penal Code. V.T.C.A. Penal Code Sec. 9.81(a).1
Appellant testified that he had known Menchaca for about four years and that there was ill will between the two. He said he feared Menchaca, and that he was afraid Menchaca would beat him. His fears were justified, he said, when Menchaca twice assaulted him on the day of the alleged offense.
Appellant’s attorney tried to elicit evidence supportive of his theory of the case while cross-examining Menchaca. In the course of this cross-examination Menchaca admitted that he knew one Oscar Antu, but categorically denied that he had ever been involved in any fight with Antu.
Later, appellant’s attorney called Antu to testify. The record reflects by examination of the witness outside the presence of the jury that Antu would have testified that deputy Menchaca without provocation had beaten him in the Maverick County jail.
The trial court erred in excluding this testimony. It is fundamental that when a witness in a criminal case testifies about a specific fact or event, and that fact or event is more than a very minor detail of his testimony, then the opposing side may present evidence to rebut the testimony. Such impeachment goes directly to the credibility of the witness, a factor that in many cases may critically affect the outcome of the prosecution. E. g., Daley v. State, Tex.Cr.App., 491 S.W.2d 932; Simons v. State, 167 Tex.Cr.R. 15, 317 S.W.2d 740; Freeman v. State, 166 Tex.Cr.R. 626, 317 S.W.2d 726; Redding v. State, 161 Tex.Cr.R. 53, 274 S.W.2d 712 (on motion for rehearing). The right to impeach the prosecution’s witnesses is also one aspect of the Sixth Amendment right of confrontation. See, e. g., Davis v. Alaska, 415 U.S. 308, 94 S.Ct. 1105, 39 L.Ed.2d 347 (1974); Napue v. *95Illinois, 360 U.S. 264, 79 S.Ct. 1173, 3 L.Ed.2d 1217 (1959).
Nor can we conclude that the error was harmless. The question of guilt was fiercely contested and hinged primarily upon the relative credibility of the witnesses. Moreover, the point upon which appellant sought to impeach the complaining witness was critical to his entire defense.
For failure of the trial court to allow appellant to impeach the testimony of the complaining witness, the judgment is reversed and the cause is remanded.
. “(a) ... a person is justified in using force against another when and to the degree he reasonably believes the force is immediately necessary to protect himself against the other’s use or attempted use of unlawful force.”
We note that appellant several times tried to testify as to all the circumstances contributing to his alleged reasonable belief that force was immediately necessary to protect himself against Menchaca’s use or attempted use of unlawful force. Without reaching the merits of a ground of error predicated upon the exclusion of such testimony by the trial court, we feel constrained to state our opinion that under the above quoted statute a defendant is entitled to present evidence relevant to such belief.