The Soldiers' Triangle is a not actually a triangle at all. Rather, it is a piece of doctrine, fed to impressionable recruits who will then believe it. However, that does not stop it being a fairly true and useful teaching idea. On each side of the triangle is one of the things that will make a soldier a dangerous force on the battlefield. However, is one of the sides of the triangle fails, it all falls apart, so there is strength in balance.
As my ASCII art is appalling, I won't try to draw a triangle for you. You will have to imagine one. Imagine a nice equilateral triangle. It doesn't matter what it looks like, just as long as it is a vaguely triangular triangle. On each side, put a label. One of the sides is labelled 'Firepower', another 'manouverability', and the third 'Survivability'. Although the triangle can be stretched and twisted a little, if it is done too much it will collapse, and your soldier/tank/whatever becomes useless.