Would Mexico Have Viewed a Mexican Advance North of the Rio Grande as a U.S. Invasion?

The question contains a logical inversion. A Mexican advance north of the Rio Grande would be Mexico moving into territory the U.S. claimed—not a U.S. invasion.

Historical Context

• 1836: Texas declares independence; claims Rio Grande as border.
• 1845: U.S. annexes Texas; inherits Rio Grande claim.
• 1846: Mexico disputes border, says it's the Nueces River (150 mi north). Mexican troops cross Rio Grande → U.S. declares war.

Mexican Perspective (1840s)

• Mexico considered Texas Mexican territory (never ratified independence).
• Any Mexican military movement south of the Nueces was internal policing, not invasion.
• Crossing the Rio Grande was seen as reasserting sovereignty up to the true border (Nueces).

U.S. Perspective

• Annexation made Rio Grande the U.S. border.
• Mexican troops north of Rio Grande = invasion of U.S. soil (casus belli).

Answer

Mexico would not view its own advance as a U.S. invasion. It would view it as defending/reclaiming Mexican territory. The U.S. viewed the same act as an invasion of the U.S. This irreconcilable border dispute caused the Mexican–American War.