Prior to the "nuclear option", I really do not think there was a "button" so to speak that the President of the United States had at his disposal, save one option - and by "button", I mean the same thing I think you do - as in, "what very rapid course of action was available for a President to either defensively retaliate or offensively strike an enemy of the state on their own, without Congressional approval?"

In the US, Congress is supposed to be the only entity that has the ability to officially declare war on another nation. However, I believe the closest thing to the "button" would be the President's capability to send troops into hostile/combat action without Congressional approval for a period of 60 days (with a further 30 day withdrawal period). That would currently be per the War Powers Resolution of 1973 (a power which existed prior to that per the US Constitution's Article II Section 2 powers), which requires the President - also the Commander In Chief of the Armed Forces - to notify Congress within 48 hours of committing US troops to combat roles, but allows for that 60 day window of opportunity afterwards to allow those combat forces to engage in military actions against an enemy without direct Congressional approval.

There are some apparent politically exploitable holes there however worth pointing out. After all, the US has not officially declared "war" against another nation state since WWII, though, for example, the Korean War, the Vietnam War, the two Gulf Wars with Iraq, and the current ongoing "War On Terrorism", by any reasonable examination, seem to fit the logical bill of showing US troops committed to engaging in "warfare" that extended beyond the 60-day mark (heck, even the Presidents and Congress who committed those troops to those conflicts consistently referred/refer to those military actions publicly as "wars" by name, even though they also claim they were not legally declarations of "war"). One way that has been shown to try to circumvent the War Powers Resolution is for a President, focusing typically on Article II Section 2 of the Constitution (which states that "The President shall be Commander-in-Chief of the Army and Navy of the United States."), to not, strictly speaking, ask Congress for a declaration of "war", but rather refer to the clear US military combat involvement as being a "police action" or a "peacekeeping" force. In the first Gulf War in 1991 after Iraq invaded Kuwait for example, that combat action was never "officially" referred to as a "war" under HJ resolution HR-77 (the "Authorization for Use of Military Force Against Iraq Resolution"), but was rather deemed to legally be an international military action to "restore international peace and security in that area" with UN support, with no mention of the word "war" at all. Article 51 of the UN charter also allows a nation the inherent right to engage in self defense actions if they are attacked first militarily, without having to legally declare war to do so.

So, probably the best answer I can think of for you, prior to the nuclear option, would have been the Presidential capability to send troops into combat against a foreign adversary immediately and at their sole discretion as per their interpretation of Article II Section 2 of the Constitution, with a 60 day window allowance for combat action, where they do not have to require Congressional approval to do so, to best fit the bill.