At a bit of a standstill over the battle for July.
Tons of "decisive" battles in July for the American Civil War: Vicksburg and Gettysburg in 1863, Atlanta in 1864. But I have a hard time seeing the battles as decisive in and of themselves. Perhaps Vicksburg, but then, the entire campaign really led to the fall of the City. There are good arguments that the battle at Champion Hill, fought in May, really broke Pemberton and set up the collapse two months later. Gettysburg is really a replay of Antietam; Lee might have won if he'd let Meade come at him, but he didn't, and the Army of Northern Virgina was never really able to get over its notion of itself as the Shield of the Confederacy. It fought like hell in defense, but always seemed to lose its focus when it crossed the Potomac.
One "decisive" battle I'd hate to do but think that - along with Verdun, which is my choice for next February - is important in history is the first month of The Somme. The decisive nature of the battle wasn't on the ground but in the minds of the English generation of 1916 and generations thereafter. I'd argue that even today, if you try and propose to many English or French the notion of a hard-fought war you have to argue against the memory - however many times removed - of the pointless slaughter of the Somme and Verdun. Of the two, though, I think Verdun was by far the more terrible, and it had an effect on France much graver than the Somme had on England. So perhaps the one can stand for both, and of the two, the Meuse Mill would be my choice.
So...any candidates for the next two days - it's not too late! Vote for your favorite battle in July before the dog days of August commence...