How we change what others think, feel, believe and do
Attack your enemy when they are not expecting it.
Sneak up on them or otherwise cloak your actions so they have no time to get
ready to defend themselves.
Attack from multiple directions. These may be staggered such that when they are rushing to defend against the first attack, another attack is launched against them.
Surprise can also be achieved from afar, such as in bombing raids or artillery fire.
The surprise attack may have several purposes. When it is launched using a larger force, the intent is defeat. A smaller force that could be easily overwhelmed by the enemy may be used either to sow seeds of confusion or fear or to achieve a particular mission such destroying an arms dump or freeing captured troops. In any case, the small force retreats soon after attack.
Surprise often causes confusion and it is a mark of an army's discipline as to how it reacts to a surprise attack. If they are well-organised, they will respond quickly. You can thus use a small surprise attack to test them. Repeated surprise attacks, however may have diminishing value as they prepare more and are surprise less.
The 1941 attack on Pearl Harbor by the Japanese in World War 2 was totally unexpected by the Americans. For the Japanese it was a huge success, killing 2403, injuring 1178 and destroying or crippling 5 battleships, 3 destroyers, 3 cruisers, and 188 planes. Japan lost 64 people, 29 planes and 4 submarines.
Use surprise in an argument to throw them off track, for example by bringing in a surprise witness. Then as they fumble you can demand particular concessions.