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The arrival of the remainder of Manticoran 3rd Division, including 3rd Cavalry Brigade and 10th Infantry Brigade, and the decision made by the Argentines to cross the Parana and Uruguay River deltas for a land-based offensive against the Manticorans marked the effective end of the Montevideo campaign and the beginning of the Sacramento campaign, as it later became known. Although the size of the Argentine invasion force was massively increased by the patriotic fervor the Argentines felt in the wake of their defeat of the British, their force was still based primarily around militia, although Spanish regulars and marines did make up a fairly large element. The Manticorans, now with more than 17,000 troops on the north bank of the Rio de la Plata, were outnumbered, but the correlation of forces was helped by the fact that the Manticorans were extensively trained and well equipped professionals. Using small boats and barges to cross the expansive swamps, the Argentines landed near the Spanish fortress of San Lazaro at the mouth of the Uruguay. About 130 miles from Montevideo, the Argentines were about two or three weeks march from their destination. Manticoran cavalry and scouts quickly discovered the approaching army and went to work attacking Argentine scouts and foraging parties. For their part in this extended action, 6th Hussars was awarded the appellation “Royal”.
Still far from their destination but in urgent need of supplies, the Argentines approached the old Spanish town of Colonia de Sacramento, where they hoped to receive cargo smuggled across the estuary from Buenos Aires. The Manticoran infantry, less some companies left to garrison Montevideo and the surrounding area, quickly responded, approaching the town from the east while the Argentines moved up cautiously from the west. The opposing forces met on opposite banks of a small, swampy creek that flowed out of the hills north of Sacramento. The Manticoran grenadiers were held in reserve while the line infantry crossed the stream under heavy fire. However, the weight of Manticoran artillery eventually told, forcing the Argentines back and allowing the infantry to complete their crossing. In this action, 10th Line on the Manticoran left received the appellation “Guards” for their intense pursuit of the retreating Argentines. The Lancers followed up this success with their own attack, pushing forward against the collapsing Argentine lines in echelon with the advancing infantry. Here, 2nd Lancers was awarded the appellation “Guards” for their reckless attacks that broke the Argentine right flank. With the moment of decision near, 3rd Grenadier Brigade advanced toward the new apex of the Argentine line, where the collapsing right met the standing left. The Grenadiers’ and Fusiliers’ rapid assault began to roll up the Argentines, allowing the 9th Infantry Brigade to get their own assault back underway. Although the Hussars and Dragoons accompanying the Manticoran field army were itching for pursuit, nightfall eventually allowed the Argentines time to retreat.
When the Manticoran main body caught back up to the Argentines two days later, it was only because the Argentines had stopped to establish defensive positions behind the San Juan River, about a third of the way back to their landing points. Significant numbers of Argentine militia had already left, either having been released from service on account of unexpected casualties and supply problems or outright deserting. With only a few thousand Spanish regulars and marines remaining in the field, most of the Manticoran infantry returned to their occupation duties while 3rd Grenadier Brigade, with significant cavalry support, maintained positions across from the Spanish. Although the Grenadiers were planning their own assault across the river, the entire war with Spain was interrupted by Napoleon’s removal of Charles IV from the Spanish throne and the beginning of serious Spanish resistance against the French. The Spanish regulars quickly asked not for a Manticoran departure from Montevideo, but for transport back to Buenos Aires, effectively leaving the Manticorans in control of the Banda Oriental. The large force committed by Manticore to the campaign, equivalent to roughly a third of the north bank’s sparse population, and the heavy casualties suffered during the battle at Sacramento made the Manticorans unwilling to simply hand the territory back.