In the Middle Ages, war was typically a personal matter between kings, being fought by those with a personal loyalty to their superior. Civilians had little or nothing to do with these wars, and so Christian charity towards noncombatants usually prevailed, and in Europe, this attitude mostly existed until the end of the First World War. Mass national democracy changed this attitude: the enemy was no longer a single individual, but an entire undifferentiated peoples. This also changed the definition of patriotism: no longer would you love your country simply because it is yours.
See the article Dehumanizing the Enemy; Mass democracy, mass warfare, mass barbarism by Andrew Cusack.
The increase of popular power... meant that war now had to be “sold” to the masses in order to ensure their support and prevent domestic unrest. Reasons for war that may have been convincing enough for the upper echelons may not have been convincing enough for the masses to whom they had partly abdicated their authority. The simple war of A versus B was replaced with [that] of a war between Good and Evil. The result of this demagogic shift in concept was for humanity to be restricted to our own side and the enemy to be downgraded to subhuman status.
Patriotism then was conditioned on the belief that one's own country was good and not merely the fact that it was one's own. More dangerously, this led to the belief that those who were patriotic to a bad country were defective. The vast increase in democracy between the World Wars, which led to the change in the definition of patriotism, inevitably led to a new kind of warfare:
The very apotheosis of this dehumanization, however, was during the Second World War. The Nazis (partly inspired by American eugenicists) attempted to create a science of determining which men were humans and which were subhuman. In the disgusting process, millions of not only the conquered but even their own German citizens were helplessly slaughtered in consequence, while the citizens of London and other cities were forced to brave the Blitz. The Anglo-American response was scarcely any better. The RAF Bomber Command employed scientists to determine how best to inflict harm upon the enemy populace, not the enemy armies. The result was the firebombing of large urban centers, including purely residential districts, with the directed aim of creating firestorms powerful enough to suck the air from basement raid shelters, thus suffocating innocent women and children in their only protected place.Although improvements in weaponry and a weak spirit of charity has greatly reduced civilian casualties in some recent warfare, the false new definition of patriotism has remained. Nowadays, some claim that a person must accept national political decisions in order to be patriotic, while others condition their patriotism on national virtue.
Love that conditions itself on the behavior of others is not love at all. Imagine a child, who tells a parent that he does not love him unless get gets an expensive toy, or a parent who will only love a child if she is good. Telling others that they don't love you unless they do something is manipulation and most likely sinful; you can't force someone to love you.
Even someone who is not patriotic, who hates her country as an abused child hates her family, ought to realize that others may not share her opinion. But sometimes they will use national merit as a condition of their own patriotism, which as we have seen, isn't patriotism at all. This leads such a person to the dangerous belief that those who are patriotic to an unworthy country are sub-human, which has many terrible consequences.