In my mind it's important to place the battle in the context of the larger campaign.

In July of 1861 there was a small fight near Mesilla which allowed the Confederates to declare the territory of Arizona basically covering the southern half of modern New Mexico and Arizona. But the Confederates had a real problem, there were a lot more Union forces in the territory.

None the less General Henry Sibley had big plans, first he was going to march all the way up to Ft Laramie in modern Wyoming and cut off the Oregon Trail. Then he was going to go back to New Mexico and march over to San Diego and open a Pacific port for the Confederacy. All of this he planned to do with troops he would recruit on the way and supplies he would capture at Union forts. It was nothing if not audacious.

Sibley detached a small unit over to Tucson to block any advance from California (more on them later) and started immediately north. His first obstacle was Colonel Edward Camby at Fort Craig. He had to take the fort in order to advance up the Rio Grande. So he marched up to Fort Craig. Camby outnumbered Sibley about 3,000 to 2,590. But most of Camby's men were relatively untrained. None the less he was game and took on Sibley in a set piece battle at Valverde Ford. The fight was a technical Confederate victory. But the Union men were able to retreat to the fort.

Seeing how strong Fort Craig looked, Sibley decided not to attack it, but instead to advance up the river. Camby disbanded his militia and volunteer units and formed up into a fast cavalry force. He intended to harass Sibley from behind and ahead with skirmishers, and keep Sibley from foraging or receiving supplies from down river. Sibley largely wasn't able to counter this because he had lost a disproportionate number of horses at Valverde. Sibley thus advanced very slowly. Camby was able to destroy or run away with most supplies in Albuquerque and Santa Fe. But his cavalry wasn't big enough to try to take on Sibley in another battle.

In the meantime Union forces were gathering at Fort Union northeast of Santa Fe. Sibley had to garrison the towns he had taken and there was the standard attrition. His starting force of 2,600 was down to 1,100. But Fort Union beckoned. If he could capture that, he'd have the materiel to continue on. The problem is at this point there were a couple thousand Union soldiers at the fort. Over 1,300 of these marched south to fight Sibley under the command of Colonel John Slough who had just arrived from Colorado with reinforcements.

Sibley realizing this sent an advance guard of 300 men to hold Glorietta Pass. Slough had a similar idea and sent Major John Chivington to do the same thing. The Union forces won the skirmish on March 26th. Chivington camped just south of the western enternce to the pass. Lt Colonel William Scurry brought up the bulk of the Confederate forces, while Slough brought up the Union.

So on the morning of the 28th the positions were more or less as follows. Scurry had 1,100 men on the west end of the pass. Slough has somewhere around 700-900 men (eyewitness accounts vary). Chivington was sent on a flanking maneuver. And both Scurry and Slough advanced up their sides of the pass. The Confederates had more men and better positioning and were able to force Slough than to retreat. The Confederates decisively won the battle. But Chivington with his 430 men were now in the Confederate rear, trying to find a flank that has moved on. Instead they find the supply train. So Chivington drives off guards takes what he can, burns or kills the rest and turns back to find Slough. All in all 80 wagons were destroyed and some 500 horses and mules were killed and driven off.

Despite having won the battle, without supplies the Confederates can't advance. They fall back to Santa Fe, then they make a terrible retreat into Texas harassed by Camby the whole way. Sibley loses hundreds more men on the retreat.

In the meantime a force known as the California Column (2,300 men strong) was advancing across southern Arizona handily defeating the small detachments in their way, but slowed by raiding and supply problems of their own. None the less by mid summer they had reached Sibley's supply bases. At the same time public opinion was turning decidedly pro-union with militias popping up across southern New Mexico. These militia proved to be the final straw as they met Sibley at the Second Battle of Messila and drove him into Texas.

Even if he had kept his supply train, post Glorieta, Sibley would have had a larger army in front of him, a larger army approaching through Arizona, several well manned forts like Fort Craig he hadn't taken down yet, Camby's cavalry, and the equivalent of a popular uprising. Win or lose it's hard to imagine Sibley would have gotten much further than he did. But he did lose, so his campaign completely collapsed.