I point you to 10 USC Section 1034, which says, in part:
10 U.S. Code § 1034 - Protected communications; prohibition of retaliatory personnel actions
(a) Restricting Communications With Members of Congress and Inspector General Prohibited.
(1) No person may restrict a member of the armed forces in communicating with a Member of Congress or an Inspector General.
(2) Paragraph (1) does not apply to a communication that is unlawful.
(b) Prohibition of Retaliatory Personnel Actions.
(1) No person may take (or threaten to take) an unfavorable personnel action, or withhold (or threaten to withhold) a favorable personnel action, as a reprisal against a member of the armed forces for making or preparing or being perceived as making or preparing
(A) a communication to a Member of Congress or an Inspector General that (under subsection (a)) may not be restricted;
(B) a communication that is described in subsection (c)(2) and that is made (or prepared to be made) to
(i) a Member of Congress;
(ii) an Inspector General (as defined in subsection (j)) or any other Inspector General appointed under chapter 4 of title 5;
(iii) a member of a Department of Defense audit, inspection, investigation, or law enforcement organization;
(iv) any person or organization in the chain of command;
(v) a court-martial proceeding; or
(vi) any other person or organization designated pursuant to regulations or other established administrative procedures for such communications; or
(C) testimony, or otherwise participating in or assisting in an investigation or proceeding related to a communication under subparagraph (A) or (B), or filing, causing to be filed, participating in, or otherwise assisting in an action brought under this section.
(2)
(A) The actions considered for purposes of this section to be a personnel action prohibited by this subsection shall include any action prohibited by paragraph (1), including any of the following:
(i) The threat to take any unfavorable action.
(ii) The withholding, or threat to withhold, any favorable action.
(iii) The making of, or threat to make, a significant change in the duties or responsibilities of a member of the armed forces not commensurate with the members grade.
(iv) The failure of a superior to respond to any retaliatory action or harassment (of which the superior had actual knowledge) taken by one or more subordinates against a member.
(v) The conducting of a retaliatory investigation of a member.
There is more, but you get the point. Its illegal to interfere with a soldiers communication with Congress.