Liberals don't understand that Rome's goal in the First Punic War was never to kill Carthaginians; they wanted to control Sicily. The Romans never wanted to kill the Samnites; they just wanted to control southern Italy. Alexander the Great probably did just want to kill as many Persians as possible... but that's just him as an individual; Macedonia wanted to control the vast territory and riches of the Persian empire. Does this distinction make sense?
Fundamentally, liberals see war as a game played by two different teams. They get angry if a civilian is killed because that guy didn't agree to play. What's supposed to happen is that both sides just exhaust their armies and sign a treaty like gentleman who made a bet on a game of skill. For the liberal, Caesar went to Gaul to kill Gallic armies and then he won control of the territory as a prize at the end. In reality, he'd much prefer there have been no Gallic troops to be found, and just skip right to the territory and riches.
This is why liberals are completely unable to see what Mexico is doing for what it is: An invasion of the US. In my last paragraph, I felt a little bit ridiculous even imagining that Caesar could walk right into Gaul and find zero resistance, but that's the exact scenario Mexicans find themselves in today. They are walking right in and they walk off with our riches and unofficially with our territory, notably parts of Texas and California.
Liberals can't see this though, because the game isn't being played "properly". There's no military. There's no formal declaration of war. There's no battlefield. This is just not how the "game" is played. When you point out an invasion to a liberal, they think of it as if you're accusing those Mexicans of playing football when there isn't a football field or helmet in sight. Ultimately, they don't see war as clashes between civilizations. They just have some childish image of two commanders agreeing to fight on a particular day, on a large freshly mowed battlefield, and probably some flaming arrows. They don't know what conflict is, and so they don't know when to protect themselves.