All's fair in love and war
They don’t call it war because it’s a good way to settle disputes.
Wars are devastating fights between two or more factions that throw all morality and decency out the window in pursuit of the ultimate prize. People have the understanding that they may very well perish when they’re sent out to fight, but they do so with great dignity so that they can be seen as honorable and courageous to their countrymen. Troops will often resort to using tactics that are anything but dignified, however, because nothing is civil about war.
Genghis Khan, in a large scale attack against the fortress at Volohai, after being unable to breach the walls demanded 1000 cats and 10,000 Chinese Swallows in exchange for lifting the siege. When the defenders sent out the animals Genghis took them and tied tufts of cotton to their tails and ignited them, at which point the animals rushed back to their homes within the city, igniting hundreds of small fires, while Genghis’ army attacked again and took the fortress amidst the panic (Eiyse)
Not a Drop Shed
When Timur the Lame was about to attack the city of Sivas, he promised the 3,000 Christian Armenian sipahis that he would not shed any of their blood if they surrendered.
They did, and, true to his word, he had them all buried alive. (Phrossack)
A Deadly Distraction
Not sure if this counts as a ‘trick’, but Napoleon prior to the Battle of Marengo.
Not only did he cross the Alps with his entire army (a completely unexpected move), but once across, the Austrians thought Napoleon would move to Genoa in order to relieve the siege of that city. Thousands were dying of starvation, and French general Massena desperately called for Napoleon’s aid.
Napoleon, in a ruthless and surprising move, ignored the calls for help and instead moved to attack the main portion of the Austrian army, eventually winning the Battle of Marengo..
Massena surrendered Genoa and it would be years before he eventually forgave Napoleon for abandoning him (and it would be years before Napoleon forgave Massena for surrendering). (lebeast)