G.W. Smith's unique account of Joe Johnston's motive for an attack on Union forces has some odd backup ... from James Longstreet, no less. In The Seven Days, Cliff Dowdey portrays Johnston and Longstreet as liars who filed crooked battle reports to do Smith dirty, and in his memoirs, Longstreet shows hostility to Smith.
He does make this interesting comment to introduce his discussion of Johnston's attack order at Seven Pines. Noting the May 15 gunboat attacks on Chapin's and "Drury's" bluffs, Longstreet says, "That attack suggetsed to General Johnston that he move nearer Richmond to be in position to lend the batteries assistance..."
This is not an exact match to Smith's claim that fear for the capital from the water triggered the assault on McClellan, but it takes things a step closer by linking Johnston's repositioning not to McDowell's movements but rather to the US Navy's movements.
Longstreet then links a decision to attack to McDowell's movements. "He prepared to attack McClellan before McDowell could reach him." This does not give us a motive but an incentive. The proposed attack fizzles in a council of war on May 27, 1862, with everyone (Longstreet says) favoring the attack except Smith, who would have led the main column. "General Johnston replied [to a comment by Longstreet] ... that he had selected the wrong officer for the work. The news of McDowell's movements passes away as they all learned he was headed for the Valley and on the 30th, Longstreet "found Gen Johnston ready to talk over plans for battle."
Up to the 27th and the council of war, Smith was the second in command and designated to do the heavy lifting in the coming attack. After the 27th, Longstreet was in the ascendant. It is likely that Smith, up to the 27th, was intimate with Johnston and his plans and ideas. He was best positioned to supply a motive, which Smith did later in life.
We can elaborate on that attributed motive in a speculative way. The riverine concerns Smith and Longstreet report Johnston as having for Richmond (not reported in Civil War histories that I've seen) would have been aggravated by any reported movement by McDowell to join McClellan. This would have represented a double envelopment of the Rebel position and to a general who perennially worried about flanking marches, this would be double trouble indeed.
In his memoirs, Longstreet gives no motive for the attack that led to the battle. He simply says JJ was "ready to talk over plans for battle."
Longstreet does not contradict Smith; he does add a little more support to Smith and the least reported genesis for Fair Oaks / Seven Pines.