The Art of War was written by Sun Tzu is a text explaining military strategy as a guide to victory.
Sun Tzu lived in the state of Wu during the Warring States period. The state of Chu became hostile towards Wu and had over 100,000 troops compared to Wu's 30,000 troops. Sun Tzu believed that discipline was a key factor in having a strong military force. He gathered a group of palace ladies and ordered them to get into formation when drums began beating. The ladies laughed and Sun Tzu explained the instructions agian in a clear manner. When the ladies laughed and did not move again, he executed one of the ladies and ordered them to start again. At this point, the ladies got into formation without hesitation. Sun Tzu was then appointed the commander of the Wu forces. At the start of the war, Sun Tzu did not embrace for the Chu's large army as he did not have the numbers. Instead of attacking the strong points, he decided to attack the weak points. He invaded Chu and destroyed Chu's remote outposts and maneuvered to avoid Chu's reinforcements. In other words, Chu's big army was going backwards and forwards only to find that Sun Tzu's forces had disappeared. Sun Tzu's tactics had frustrated Chu's forces with many defections made towards Wu. Chu decided to punish and invade Cai, a nation north of Chu. The duke of Cai called for it's ally Wu for help as they did not have the force to hold back Chu. Sun Tzu deployed a small force towards Cai which is used a bait that drew the Chu forced away from Cai and so that meant that Cai was saved without a single battle. The small force confronts the main Chu forces while Sun Tzu's main forces head to the Ying, Chu's capital. The commander of the Chu's main forces decided to chase Sun Tzu's main force otherwise if the commander of the Ying's commander held off Wu's main force then he will take credit. The commander of Chu's main forces was arrogant which was never part of Sun Tzu's thinking. As Chu's main forces gave chase, Sun Tzu turned around and surrounded them with the smaller force to catch up and attack their rear. Sun Tzu's 20000 elite troops ambushed the Chu main force by surprise and won. His movement towards Ying was deception that attracted Chu's main force. After the war Sun Tzu disappeared as he did not like his leaders' behaviour and the King of Wu enjoying the wealth he has when entering Ying. So how did an army of 30,000 defeated and army of over 100,000? Sun Tzu used preparation to discipline his troops, deception to lure the Chu forces and indirect attacks by attacking weak points and never confronting Chu head on.
So does Sun Tzu's Art of War apply to wars being fought today or the past 200 years?
Lets look at the American Civil War for example. During the Battle of Gettysberg, the Confederates attacked the Union troops sitting on top of Cemetery Hill. The Art of War advised that it is not wise to attack forces sitting on top of a hill. During the Civil War, the battles were often fought with both sides confronting each other head on which caused heavy casualties while the Art of War advised that attacks should be made towards the weakest points, in other words, Sun Tzu only wanted to use minimal resources.
During World War 2, the Art of War was applied on the D-Day invasion. Nazi Germany knew that the Allies were going to invade Europe soon and it definately around Northern France as it was in range of air support provided by planes stationed in Southern England. The Allies did some deception tactics prior to the D-Day invasion. Fake tanks (inflated) and equipment were used as fake intelligence and to make the Germans think that other beaches, not Normandy, would be invaded. During the invasion, the Allies were successful while the Germans failed. This was because of generals had no interference from their leaders. The Allies had a simple command structure with Dwight Eisenhower being the top Commander with easy access to other commanders. The Germans had a complex command structure with Hitler being the top and it meant that Hitler was the one making the final decision. The Field Marshal was serperated from the Navy and Air Force while the SS was under Himmler's command. This complex command structure was completely opposite to the Art of War.
During the Vietnam War, General Vo Nguyen Giap had followed the Art of War. He did not fought against the Americans head on because he knew his forces and the enemy just what the Art of War stated. Instead, he chose to conduct guerrilla warfare and to outthink the US instead of outfighting them. The US dropped 7 million tonnes of bombs on Vietnam relying on sheer military force to advance which Sun Tzu does not recommend. Giap knew the US would launch airstrikes before sending troops in. Giap ordered his forces to take shelter during the airstrikes and ambush the Americans when they land. Giap even ordered his forces to fight closely with the Americans to prevent US air support from bombing it's own men. Giap constantly harassed US troops via ambush, booby traps and snipers to the point where the American public no longer supported the war. In other words, the US were losing the war in Vietnam because of it's support lost from it's people. Giap subdue the US without any head on/direct fighting. Giap's advisors got impatient and demanded a full scale head on attack against the US troops. Giap decides to launch all attacks at all South Vietnamese cities and bases at the same time and kept the plan secret and launch the operation by surprise which was called the Tet Offensive. Giap even used spies prior to the offensive. Giap promised a cease fire during the Tet New Year and made a direct attack on Khe Sanh. He wanted the direct attack on Khe Sanh to be a destraction while the indirect attack on all South Vietnamese cities and bases to win the war. However, during the Tet Offensive, the Viet Cong hoped the people of South VIetnam rose and help them win but the massacre of South Vietnamese sympathizers in Hue saw the people of South Vietnam refusing to give support. The images of the Tet Offensive saw the increase in support from the American public. Giap returned to his typical guerrilla tactics which led to anti war protests in the US. After US withdrawal, Saigon finally fell to North Vietnam.