die Schlacht
battle
A large, organized fight between military units as part of a war.
In der Schlacht bei Leipzig kämpften mehrere hunderttausend Soldaten.
Several hundred thousand soldiers fought in the Battle of Leipzig.

battle / slaughter
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battle
A large, organized fight between military units as part of a war.
In der Schlacht bei Leipzig kämpften mehrere hunderttausend Soldaten.
Several hundred thousand soldiers fought in the Battle of Leipzig.

'Die Schlacht' (the battle/slaughter) is closely related to the verb 'schlagen' (to hit, to beat). At its core, a battle is a large, organized form of 'hitting' between two groups.
Die beiden Heere schlugen eine blutige Schlacht.
The two armies fought a bloody battle.
In a military context, 'Schlacht' (battle) is often combined with certain verbs. Typical phrases are 'eine Schlacht schlagen' (to fight a battle), 'eine Schlacht gewinnen' (to win a battle), or 'eine Schlacht verlieren' (to lose a battle).
Die Armee hat die entscheidende Schlacht gewonnen.
The army won the decisive battle.
In a figurative sense, 'Schlacht' (battle) describes an intense fight. It is often used in compound words, which can also be playful, such as 'Kissenschlacht' (pillow fight) or 'Schneeballschlacht' (snowball fight).
Die Kinder hatten im Garten eine lustige Schneeballschlacht.
The children had a fun snowball fight in the garden.
The meaning 'killing animals' is rare today when the word stands alone. You almost always recognize it in compounds such as 'Schlachthof' (slaughterhouse), 'Schlachttier' (animal for slaughter), or 'schlachten' (to slaughter, the verb).
Der Metzger arbeitet auf dem Schlachthof.
The butcher works at the slaughterhouse.
war
'der Krieg' and 'die Schlacht' both belong to the military sphere, but they do not mean the same thing. 'Krieg' is the whole armed conflict, which can last a long time and involve many places. 'Schlacht' means a large, individual battle within a war. Learners may confuse the words because both have to do with armies and fighting. The important point is: a war can have many battles, but a battle is only one part of a war.

Die Schlacht fand an einem kalten Morgen vor der Stadt statt.
The battle took place on a cold morning outside the city.
Der Krieg zwischen den beiden Ländern dauerte fast fünf Jahre.
The war between the two countries lasted almost five years.