The term flank means to be to the side of anything. If soldiers are flanked, they are split and one group goes to one side of the object and the other group goes to the other side of it. In the case of a fire, the flanking fire is commonly used to execute fire to the soldiers from the side. This may be done in order to be a surprise attack.
Sometimes, wildfires can flank. In this case, the fire branches off to the side of something as if it is traveling around an area. A flank fire is a procedure in which firefighters will light a fire on either side of the wildflower in the hopes that this fire will actually put out the wildfire that is raging out of control.
