Combat indeed took place in New Mexico Territory. The Confederates staged an invasion in 1861, and the Confederate Territory of Arizona was admitted to the Confederacy in 1862. Of course, the U.S. government had recognized the pro-South sentiments of the southern portion of New Mexico, and so had divided the territory along the north-south border we're familiar with today (creating two western states instead of another northern and southern rivalry). The invaders initially met with success, as they took the town of Mesilla and won a small battle with Union forces (total casualties of both sides: ~25). Their plan was to use New Mexico Territory as a staging ground to take Colorado's and California's gold fields, as well as California's Pacific ports.

The Confederates were led by Brig. Gen. Henry Hopkins Sibley, a Louisianan who had attended West Point and served with distinction in the Mexican-American War. He also had extensive experience in New Mexico, having led dragoons against the Navajos before the war. When news of war breaking out reached the Territory, he resigned his commission and headed for El Paso, telling his loyalist colleagues, "Boys, if you only knew it, I am the worst enemy you have!" He also had such a heavy drinking habit that his troops nicknamed him "the Walking Whiskey Keg" and he was frequently confined to his ambulance, apparently unable to command, with bouts of "colic."

He was opposed by his former C.O. in New Mexico, Col. Edward Canby, although their relationship was deeper than merely professional acquaintance: Canby had been best man at Sibley's wedding, where he married Canby's wife's cousin. Also a West Pointer (though not necessarily the brightest one - he was 30th in a class of 31), he was a careful, taciturn, conscientious leader of Quaker heritage who had fought Seminoles in Florida, served with William Tecumseh Sherman in California, and won praise from Gen. Winfield Scott in the Mexican-American War. He was well aware of the pitiful military state of New Mexico, and in hopes of deterring Sibley from attacking his headquarters at Fort Craig directly, had reinforced it with 4,000 troops and heavy cannon.

Unfortunately, these new cannons were actually painted pine logs (called "Quaker guns" by his men) and only 1,200 of his troops were Army regulars - the rest were New Mexican militia, nearly all Hispanic, who Canby considered "worse than worthless...[every desertion] only adds to rather than diminishes our strength." While his assessment of Hispanic troops overall was undoubtedly colored by the pervasive racism of the time, not to mention the general disdain many Mexican War veterans held for the defensive skills of Hispanic fighters, he was not incorrect to question their loyalty and motivation: as author Hampton Sides put it, "[the militia's] lack of fighting experience was matched only by their lack of a concrete rationale for fighting - understandably, they had little interest and seemingly no stake in the issues of this Americano war begun the previous year in some utterly foreign place called South Carolina."

Sibley and Canby knew they would have to engage each other. Sibley wanted to get Canby's forces away from those long guns he could see through his field glasses, and Canby was terrified of losing the fort should/when his undisciplined militia break and flee from a fight. Sibley was also running desperately low on food and water. Eventually the two armies would meet at a ford on the Rio Grande called Valverde on Feb. 21, 1862.

Sibley found himself stricken with "colic" as his troops marched into battle and spent most of the fight in his ambulance. The battle is mostly notable for what is widely considered to be the only lancer charge of the Civil War - lancers had proven remarkably effective in the Mexican-American War (Gen. Stephen Kearny had nearly bled to death after his unit's encounter with mounted lancers in California) and about 50 Texans had armed themselves with nine-foot spears with foot-long blades. They boldly charged the Union lines, but at a range of only 100 yards, a Union officer yelled "They're Texans - give 'em hell!" and another Union soldier later wrote "it was fun to see the Texans fall. On they came and fierce looking fellows they were with their long lances raised, but when they got to us we were loaded again and then we gave them the buck and ball. After the second volley there were but a few of them left. One of them got away. The others were shot and bayoneted." Most of the lancers and nearly all of their horses were killed, and the survivors burned their lances and re-armed with shotguns.

Canby's New Mexican militia outperformed his expectations. While the broader political nuances of the Civil War were likely lost on most volunteers, there was one concrete fact that kept them at arms: Texans were invading. Texas, ever since declaring independence from Mexico, had claimed their border naturally extended the entire length of the Rio Grande, up to its headwaters in Colorado. They had tried to draw new county lines that would have incorporated large parts of New Mexico into Texas when they became a state. In 1841, an armed expedition even invaded New Mexico, hoping to take Santa Fe (they were quickly captured and sent to a notoriously brutal prison in Mexico City). New Mexicans saw Texans as a sort of bogeyman - an uncommonly brazen people who saw New Mexico and especially Santa Fe as rightfully theirs (despite their disdain for the land itself and the people who lived there), and who had (by their own accounts) not been shy about "appropriating" New Mexican women in towns they passed through during their invasion. While a militia drawn to save the Union might have not fought for beans, a militia formed to stave off a Texan invasion had reason to stay on the battlefield. It was only at the end of the day, with many Texans driven nearly insane by thirst, that a last, desperate charge broke the Union lines. The Texans captured Canby's best artillery unit (though a Union gunner, intending to deny the rebels his ammunition, lit a fuse on his box of shells and caused considerable destruction), and even shot Canby's beloved war horse out from under him. He ordered a full retreat back to the fort, even though several companies of New Mexican militia on his right flank, led by Col. Kit Carson, thought they were on the verge of victory. But fall back they did, and Sibley found himself the tactical victor. However, it was a strategic loss - the horses lost during the battle could not be replaced (nor could the 150 horses and mules who had earlier been stampeded by a Union spy who loaded two mules with howitzer shells, lit their fuses, and sent them running towards the Confederate camp); most his forces before the battle had been mounted - afterwards, nearly all were infantry. Sibley had not taken the fort, with its valuable stores of food, ammunition, and medicine, and now a large, well-armed, well-fed army was perched on his supply route, safely in a fort with big guns watching his starving army. Despite the disparity in their forces, Sibley demanded the fort's surrender; Canby, unsurprisingly, declined.

Despite essentially losing his supply line, Sibley pressed on. While initially believing his forces would be seen as liberators and embraced by the population, the revulsion Texans and New Mexicans shared for each other nipped this hope in the bud (remember, Sibley was from Louisiana - later, some Texans would claim they would have been successful had they been led by a Texan). They became, as Sides called them, "an army of looters." They found Albuquerque nearly deserted, with only the smoldering remains of Union supplies (Canby had sent messengers on fast horses instructing quartermasters to burn their stores rather than let the Texans have them). They pushed on to Santa Fe, which was also deserted - New Mexicans had fled the Tejanos, and the territorial government had moved 70 miles east to Las Vegas. They took Santa Fe without a shot fired and raised the Confederate flag over the plaza, which did much for the men's morale, but did not substantially change their situation.

Canby knew that Sibley's army was hurting, and had requested reinforcements from California and Colorado to finish them off. The Colorado volunteers were coming from Denver at a blistering pace - in one 36-hour period, they marched 92 miles, and the entire 400 mile march from Denver to Fort Union was completed in 13 days. After taking about a week and a half to rest, they left the fort and headed for Santa Fe, spoiling for a fight. The Texans were also eager for action, and a force of about 400 left Santa Fe when they heard the troops at Fort Union were on the move. However, they expected to face ill-trained New Mexican militia, not fresh Colorado volunteers. The Texans quickly pulled back after the aggressive Colorado skirmishers, led by Col. John Chivington, captured 71 soldiers and pushed them a mile and a half backwards. Two days later, March 28, the main bodies of the Union and Texan forces fought at Glorieta.