Beauregard led an army against Union commander Irvin McDowell and received reinforcements from Joseph Johnston's troops (whom Union General Robert Patterson failed to detain).
Anyone who has ever imagined this to be the answer to "what happened" should read the contemporary communications between Patterson and Scott in the Official Records. There is also, somewhere on the Web, an article authored by Patterson in his own defense, although Russel Beatie has pointed out that there are misleading elements in the chronologies within that article. For those who want additional insight into this, there is Patterson's testimony to the Committee on the Conduct of the War, last published in 1868 but still available through Interlibrary Loan.
In his new work, Donnybrook, David Detzer builds a Patterson defense the way I would have done it. He reviews Scott's orders ("guidance" would be a better word), looks at Patterson's disintegrating command of 90-day men, recreates a little fog-of-war, looks at statements defending Patterson's conduct (from Patterson, Pap Thomas, and even Beauregard), and declares the charge of "failing to detain" as bogus. (Pardon my violence against Detzer's clearer, more detailed argument.)
In volume one of his Army of the Potomac, Russel Beatie devoted a couple of chapters to Patterson's management of the Shenandoah side of Bull Run and worked through unique, additional sources, including private correspondence and Fitz John Porter's later defense of Patterson (Porter was adjutant to the Shenandoah army).
Beatie presents a Patterson who sensed correctly what was needed, regardless of orders and guidance; he shows that more than once Patterson publicly committed to carrying out the correct decision that he reached himself; and that more than once, Patterson revised his decision from action to inaction. Beatie's final judgement on Patterson is not whether he was right or wrong in the interpretation of his telegraphic communications, but rather that Patterson missed an opportunity to make a profound impact simply by following his own instincts.
Detzer is correct. Patterson in no way "screwed up" and it is overreaching to blame him for McDowell's defeat. Beatie is also correct, because Patterson definitely recognized an opportunity that would have advanced the cause and himself, and he shrank from that opportunity. (I think for many good reasons.)
Detzer and Beatie can both be right because Detzer is concerned with the legal/military framework for responsibility and blame, and Beatie interprets an event (and its meaning in historical time) to the nation and to the man.
"Union General Robert Patterson failed to detain" - easy to say, difficult to interpret legally, militarily, or historically.