In this sense, you could speak of the chivalry of England, meaning thereby the collective body of English knights. But chivalry came to have a deeper abstract meaning, and was used to express the proper conduct of an ideal knight.
A Christian knight had an ideal of conduct set before him, and took solemn religious vows of devotion to God, loyalty to his king, personal chastity, courtesy to women, honour truthfulness and justice; and he swore to protect the weak and oppressed. Such conduct was called “chivalrous”, that is characteristic of a true cavalier, knight. Hence chivalry today means courtesy, courage, kindness, and all that we mean by fairs-play and honourable conduct.
Is chivalry possible in war? It certainly was possible in the old days; but modern methods of warfare seem to be making it more and more impossible. What did chivalry in war mean in the Middle Ages? War in the old days was largely a matter of personal, hand- to-hand combat. A true knight would, of course, fight in battle bravely and fiercely against his foes, and would do his best to conquer or kill them; but he observed certain rules of honour in fighting, as a good sportsman observes the rule of the games he plays. These rules included the sparing of non-combatants, courtesy towards and protection of the women of the enemy, and mercy to the fallen foe. No true knight (of course there were many false knights, who broke the rules of chivalry) would mal-treat and disgrace women, or slay innocent citizens, or hit a man when he was down.
Modern warfare is no longer a personal, hand-to-hand conflict. Modern guns and high explosive shells and poisoned gas have made it the wholesale scientific slaughter of an unseen enemy. And the old rules of chivalry have become out-of-date. In the Great War, thousands of innocent non-combatants were destroyed by poisoned gas and bombs dropped from aeroplanes; thousands of women were dishonoured and killed; and thousands of wounded foes were left to die and rot where they fell.
One of the real developments of chivalry in modern warfare is the work of the Red-Cross ambulances and hospitals, which collect and care for the wounded of friend and foe. For the rest, war is a brutal business, in which very little of the element of chivalry is left.