It's a moment from the Mexican War, a war that young Lieutenant Grant thought (appropriately, it seems at this distance) was unjust, but in which he offered valuable service nonetheless, while also learning many lessons that would help him in the Civil War. The U.S. forces under Old Rough and Ready, Zachary Taylor, had moved into the outskirts of Monterrey and were beginning to squeeze the Mexican army that was holed up in the city's central plaza. After a day of heavy fighting, ammunition began to run low--and from here, I'll let Jean Edward Smith, whose captivating bio of Grant I'm currently reading, tell the story:
The brigade urgently needed to be resupplied, but sending a messenger back to division headquarters would be hazardous. Mexican musket fire raked every intersection and the air was filled with grape shot. Colonel Garland called for a volunteer. Grant said he would go. Like a trick rider in a rodeo, he hooked one foot around the cantle of his saddle, one arm around the neck of his horse, Nelly, and with his body clinging to the sheltered side, galloped away at full speed. "It was only at street crossings that my horse came under fire, but these I crossed at such a flying rate that generally I was past and under cover of the next block of houses before the enemy fired."Horse and rider reached headquarters safely, and Grant, according to friends, responded to praise with his typical modesty, appearing "to look upon Nelly's conduct as more courageous than his own."
What neither this account, nor Grant's own in his memoirs, is able to tell us is how the officers and soldiers around Grant reacted when they saw how he'd decided to approach his mission. Even knowing, as they surely did by then, what an uncannily great horseman Grant was, could they have reacted in any way other than by dropping their jaws and staring?