The Opinion discusses the constitutionality of the 60 termination requirement in the War Powers Resolution:
We believe that Congress may, as a general constitutional matter, place a 60-day limit on the use of our armed forces as required by the provisions of § 1544(b) of the Resolution. The Resolution gives the President the flexibility to extend that deadline for up to 30 days in cases of "unavoidable military necessity." This flexibility is, we believe, sufficient under any scenarios we can hypothesize to preserve his constitutional function as Commander-in-Chief. The practical effect of the 60-day limit is to shift the burden to the President to convince the Congress of the continuing need for the use of our armed forces abroad. We cannot say that placing that burden on the President unconstitutionally intrudes upon his executive powers.4a Op. O.L.C. at 196.
Finally, Congress may regulate the President's exercise of his inherent powers by imposing limits by statute. We do not believe that Congress may, on a case-by-case basis, require the removal of our armed forces by passage of a concurrent resolution which is not submitted to the President for his approval or disapproval pursuant to Article I, §7 of the Constitution.
The Opinion is also noteworthy for its discussion of what constitutes "hostilities" for purposes of the 60 day clock in the WPR:
We agree that the term "hostilities" should not be read necessarily to include sporadic military or paramilitary attacks on our armed forces stationed abroad. Such situations do not generally involve the full military engagements with which the Resolution is primarily concerned. For the same reason, we also believe that as a general matter the presence of our armed forces in a foreign country whose government comes under attack by "guerrilla" operations would not trigger the reporting provisions of the War Powers Resolution unless our armed forces were assigned to "command, coordinate, participate in the movement of, or accompany" the forces of the host government in operations against such guerrilla operations. 16 50 U.S.C. § 1547(c).
[footnote 16]: We believe that the definition of "introduction of United States Armed Forces" in § 1547(c) supports the proposition that members of the armed forces stationed in a foreign country for purposes of training or advising military forces of the host government are not generally to be viewed as subject to the War Powers Resolution. [end footnote]
Furthermore, if our armed forces otherwise lawfully stationed in a foreign country were fired upon and defended themselves, we doubt that such engagement in hostilities would be covered by the consultation and reporting provisions of the War Powers Resolution. The structure and thrust of those provisions is the "introduction" of our armed forces into such a situation and not the fact that those forces may be engaged in hostilities. It seems fair to read "introduction" to require an active decision to place forces in a hostile situation rather than their simply acting in self-defense.17
[footnote 17:] in contrast, as passed by the Senate, the bill would have required a report whenever our armed forces are "engaged in hostilities." S. 440, 93d Cong.. Ist Sess. §4, 119 Cong. Rec. 25,119 (1973). [end footnote]
4a Op. O.L.C. at 194.