Asymmetric warfare
The Filipino General Francisco Macabulos described the Filipinos' war aim as "not to vanquish the U.S. Army but to inflict on them constant losses." They initially sought to use conventional tactics and an increasing toll of U.S. casualties to contribute to McKinley's defeat in the 1900 presidential election. Their hope was that as president the avowedly anti-imperialist future Secretary of state William Jennings Bryan would withdraw from the Philippines. They pursued this short-term goal with guerrilla tactics better suited to a protracted struggle. While targeting McKinley motivated the revolutionaries in the short term, his victory demoralized them and convinced many undecided Filipinos that the United States would not depart precipitously. For most of 1899, the revolutionary leadership had viewed guerrilla warfare strategically only as a tactical option of final recourse, not as a means of operation which better suited their disadvantaged situation. On 13 November 1899, Emilio Aguinaldo decreed that guerrilla war would henceforth be the strategy. This made the American occupation of the Philippine archipelago more difficult over the next few years. In fact, during just the first four months of the guerrilla war, the Americans had nearly 500 casualties. The Philippine Revolutionary Army began staging bloody ambushes and raids, such as the guerrilla victories at Paye, Catubig, Makahambus, Pulang Lupa, Balangiga and Mabitac. At first, it seemed like the Filipinos would fight the Americans to a stalemate and force them to withdraw. President McKinley even considered this at the beginning of the phase. The shift to guerrilla warfare drove the U.S. Army to adopt counterinsurgency tactics.

20th century: 

Second Boer War: Asymmetric warfare featured prominently during the Second Boer War. After an initial phase, which was fought by both sides as a conventional war, the British captured Johannesburg, the Boers' largest city, and captured the capitals of the two Boer Republics. The British then expected the Boers to accept peace as dictated in the traditional European manner. However, the Boers fought a protracted guerrilla war instead of capitulating. 20,000-30,000 Boer guerrillas were only defeated after the British brought to bear 450,000 imperial troops, about ten times as many as were used in the conventional phase of the war. The British began constructing blockhouses built within machine gun range of one another and flanked by barbed wire to slow the Boers' movement across the countryside and block paths to valuable targets. Such tactics eventually evolved into today's counterinsurgency tactics.
The Boer commando raids deep into the Cape Colony, which were organized and commanded by Jan Smuts, resonated throughout the century as the British adopted and adapted the tactics first used against them by the Boers.

World War I: T.E. Lawrence and British support for the Arab uprising against the Ottoman Empire. The Ottomans were the stronger power, and the Arab coalition were the weaker.
Austria-Hungary's invasion of Serbia, August 1914. Austria-Hungary was the stronger power, and Serbia was the weaker.
Germany's invasion of Belgium, August 1914. Germany was the stronger power, Belgium the weaker.

Between the World Wars: Abd el-Krim led resistance in Morocco from 1920 to 1924 against French and Spanish colonial armies ten times as strong as the guerrilla force, led by General Philippe Pétain.
TIGR, the first anti-fascist national-defensive organization in Europe, fought against Benito Mussolini's regime in Northeast Italy.
Anglo-Irish War (Irish War of Independence) fought between the Irish Republican Army and the Black and Tans/Auxiliaries. Though Lloyd George (Prime Minister at the time) attempted to persuade other nations that it was not a war by refusing to use the army and using the Black and Tans instead, the conflict was conducted as an asymmetric guerrilla war and was registered as a war with the League of Nations by the Irish Free State.

World War II: Philippine resistance against Japan – During the Japanese occupation in World War II, there was an extensive Philippine resistance movement, which opposed the Japanese with an active underground and guerrilla activity that increased over the years.

Winter War – Finland was invaded by the much larger mechanized military units of the Soviet Union. Although the Soviets captured 8% of Finland, they suffered enormous casualties versus much lower losses for the Finns. Soviet vehicles were confined to narrow forest roads by terrain and snow, while the Finns used ski tactics around them unseen through the trees. They cut the advancing Soviet column into what they called motti (a cubic metre of firewood) and then destroyed the cut-off sections one by one. Many Soviets were shot, had their throats cut from behind, or froze to death due to inadequate clothing and lack of camouflage and shelter. The Finns also devised a petrol bomb they called the Molotov cocktail to destroy Soviet tanks.
Soviet partisans – resistance movement which fought in the German occupied parts of the Soviet Union.
Warsaw Uprising – Poland (Home Army, Armia Krajowa) rose up against the German occupation.
Germany's occupation of Yugoslavia, 1941–45 (Germany vs. Tito's Partisans and Mihailović's Chetniks).

Britain: British Commandos and European coastal raids. German countermeasures and the notorious Commando Order.
Long Range Desert Group and the Special Air Service in Africa and later in Europe.
South East Asian Theater: Wingate, Chindits, Force 136, V Force
Special Operations Executive (SOE)
Provisional Irish Republican Army against British security forces in the Northern Campaign.

United States: Office of Strategic Services (OSS)
China Burma India Theater: Merrill's Marauders and OSS Detachment 101.

After World War II: First Indochina War (1946-1954) and Algerian War of Independence (1954-1962); both against France
The Cuban Revolution of 1953-1958 became a template of asymmetric warfare.
The Hungarian Revolution of 1956 (or "Russo-Hungarian" war) saw makeshift forces improvising lopsided tactics against Soviet tanks.
Libyan support to the Provisional Irish Republican Army during the Troubles (1960s to 1998) and collusion between British security forces and Ulster loyalist paramilitaries.
United States Military Assistance Command Studies and Observations Group (US MAC-V SOG) (1964-1972) and Viet Cong in Vietnam.
The South African Border War, otherwise known as the Namibian War of Independence (1966-1990) between the South African Defense Force and People's Liberation Army of Namibia.
United States support of the Nicaraguan Contras (1979-1990).

Cold War (1945–1992): The end of World War II established the two strongest victors, the United States of America (the United States, or just the U.S.) and the Union of Soviet Socialist Republics (USSR, or just the Soviet Union) as the two dominant global superpowers.

Cold War examples of proxy wars: In Southeast Asia, specifically Vietnam, the Viet Minh, NLF and other insurgencies engaged in asymmetrical guerrilla warfare with France. The war between the Mujahideen and the Soviet Armed Forces during the Soviet–Afghan War of 1979 to 1989, though claimed as a source of the term "asymmetric warfare," occurred years after Mack wrote of "asymmetric conflict." (Note that the term "asymmetric warfare" became well-known in the West only in the 1990s.) The aid given by the U.S. to the Mujahideen during the war was only covert at the tactical level; the Reagan Administration told the world that it was helping the "freedom-loving people of Afghanistan." Many countries, including the U.S., participated in this proxy war against the USSR during the Cold War.

Post-Cold War: The Kosovo War, which pitted Yugoslav security forces (Serbian police and Yugoslav army) against Albanian separatists of the guerrilla Kosovo Liberation Army, is an example of asymmetric warfare, due to Yugoslav forces' superior firepower and manpower, and due to the nature of insurgency/counter-insurgency operations. The NATO bombing of Yugoslavia (1999), which pitted NATO air power against the Yugoslav armed forces during the Kosovo war, can also be classified as asymmetric, exemplifying international conflict with asymmetry in weapons and strategy/tactics.

21st century: 

Israel/Palestine: The ongoing conflict between Israel and some Palestinian organizations (such as Hamas and PIJ) is a classic case of asymmetric warfare. Israel has a powerful army, air force and navy, while the Palestinian organizations have no access to large-scale military equipment with which to conduct operations; instead, they utilize asymmetric tactics, such as paragliding, small gunfights, cross-border sniping, airstrikes, and others.

Sri Lanka: The Sri Lankan Civil War, which raged on and off from 1983 to 2009, between the Sri Lankan government and the Liberation Tigers of Tamil Eelam (LTTE) saw large-scale asymmetric warfare.