Urban warfare
Military terminology: Historically, the United States Armed Forces has referred to urban warfare as UO (urban operations), but this term has been largely replaced with MOUT (military operations in urban terrain).
The British armed forces terms are OBUA (operations in built-up areas), FIBUA (fighting in built-up areas), or sometimes (colloquially) FISH (fighting in someone's house), or FISH and CHIPS (fighting in someone's house and causing havoc in people's streets/public spaces).
The term FOFO (fighting in fortified objectives) refers to clearing enemy personnel from narrow and entrenched places like bunkers, trenches and strongholds; the dismantling of mines and wires; and the securing of footholds in enemy areas.
Israel Defense Forces calls urban warfare לש"ב (pronounced LASHAB), a Hebrew acronym for warfare on urban terrain. LASHAB in the IDF includes large-scale tactics (such as use of heavy armoured personnel carriers, armoured bulldozers, UAVs for intelligence, etc.), CQB training for fighting forces (how a small team of infantry soldiers should fight in close and built-up spaces). IDF's LASHAB was developed mainly in recent decades, after the 1982 Lebanon War included urban warfare in Beirut and Lebanese villages, and was further developed during the Second Intifada (2000–2005) in which IDF soldiers entered and fought in Palestinian cities, villages and refugee camps. The IDF has a special large and advanced facility for training soldiers and units in urban warfare.

Urban operations: Urban military operations in World War II often relied on large quantities of artillery bombardment and air support varying from ground attack fighters to heavy bombers. In the European theatre of war, roughly 40% of battles took place in urban areas. In some particularly vicious urban warfare battles such as Stalingrad and Warsaw, all weapons were used irrespective of their consequences.
Military historian Victor Davis Hanson noted the lethality of urban warfare in the Second World War, "When civilian met soldier in the confined landscapes, the death toll spiked, and it was no surprise that the greatest carnage of World War II—at Leningrad and Stalingrad—was the result of efforts to storm municipal fortresses".
However, when liberating occupied territory some restraint was often applied, particularly in urban settings. For example, Canadian operations in both Ortona and Groningen avoided the use of artillery altogether to spare civilians and buildings, and during the Battle of Manila in 1945, General MacArthur initially placed a ban on artillery and air strikes to save civilian lives.
Military forces are bound by the laws of war governing military necessity to the amount of force which can be applied when attacking an area where there are known to be civilians. Until the 1970s, this was covered by the 1907 Hague Convention IV – The Laws and Customs of War on Land which specifically includes articles 25–27. This has since been supplemented by the Additional Protocols to the Geneva Conventions of August 12, 1949, and relating to the Protection of Victims of International and Non-International Armed Conflicts.

Sometimes distinction and proportionality, as in the case of the Canadians in Ortona, causes the attacking force to restrain from using all the force they could when attacking a city. In other cases, such as the Battle of Stalingrad and the Battle of Berlin, both military forces considered evacuating civilians only to find it impractical.
When Russian forces attacked Grozny in 1999, they conducted a massive artillery and air bombardment campaign in an attempt to smash the city into submission. The Russian Army handled the issue of civilian casualties by issuing an ultimatum urging citizens to leave or be destroyed without mercy. Leaflets dropped on the city read: 'You are surrounded, all roads to Grozny are blocked...Persons who stay in the city will be considered terrorists and bandits and will be destroyed by artillery and aviation. There will be no further negotiations. Everyone who does not leave the city will be destroyed '.

Fighting in an urban environment can offer some advantages to a weaker defending force or to guerrilla fighters through ambush-induced attrition losses. The attacking army must account for three dimensions more often, and consequently expend greater amounts of manpower to secure a myriad of structures, and mountains of rubble.
Ferroconcrete structures will be ruined by heavy bombardment, but it is very difficult to demolish such a building totally when it is well defended. Soviet forces had to fight room by room while defending the Red October Steel Factory during the Battle of Stalingrad, and in 1945, during the race to capture the Reichstag, despite heavy bombardment with artillery at point blank range (including 203 mm howitzers).
It is also difficult to destroy underground or heavily fortified structures such as bunkers and utility tunnels; during the Siege of Budapest in 1944 fighting broke out in the sewers, as both Axis and Soviet troops used them for troop movements.
Analysts debate the scope and size of urban battles in the modern day, as they are unlikely to match the scale of battles in the Second World War. For example, professor Michael C. Desch states that while "enormous forces engaged on both sides in those battles may never be seen in high-intensity urban battles again", that "the large numbers of killed and wounded underline the basic fact that such conflict is extremely lethal", referencing the battles of Stalingrad and Berlin. An article by the Modern War Institute states that while lessons may be taken from Stalingrad, ultimately "Stalingrad took place in a theater with a large number of army groups with a total of a million soldiers involved on each side; modern armies are unlikely to fight with these numbers."
Many analysts, such as former American army general and Commandant of the United States Marine Corps, Charles C. Krulak, and retired military officer and chairman of the urban warfare studies at the Modern War Institute, John Spencer have predicted urban warfare to become the norm in wars. Spencer confirmed this to be true in an article in 2024, providing a list of numerous urban battles in the recent decades of the 21st century alone, those being Fallujah, Sadr City, Mosul, Raqqa, Marawi, and now Bakhmut, Mariupol, and Khan Yunis in the 2020s. In 2023, analyst Mikael Weissmann claimed that it is widely agreed upon that urban warfare will be the "battlefields of tomorrow".

Urban warfare tactics: The characteristics of an average city include tall buildings, narrow alleys, sewage tunnels and possibly a subway system. Defenders may have the advantage of detailed local knowledge of the area, right down to the layout inside of buildings and means of travel not shown on maps.
The buildings can provide excellent sniping posts while alleys and rubble-filled streets are ideal for planting booby traps. Defenders can move from one part of the city to another undetected using tunnels and spring ambushes.
Meanwhile, the attackers tend to become more exposed than the defender as they must use the open streets more often, unfamiliar with the defenders' secret and hidden routes. During a house to house search the attacker is often also exposed on the streets.

Battle of Monterrey, Mexico: The Battle of Monterrey was the U.S. Army's first major encounter with urban warfare. It occurred in September 1846 when the U.S. Army under Zachary Taylor invaded the town. The U.S. Army had no prior training in urban warfare and the Mexican defenders hid on rooftops, shot through loopholes, and stationed cannons in the middle of the city's streets. The houses at Monterrey were made of thick adobe, with strong double doors and few windows. The rooftops were lined with a two-foot-tall wall that acted as a parapet for the defending soldiers. Each home was a fort unto itself.
On September 21, 1846, the U.S. Army which included some of its best soldiers, recent West Point graduates, marched down the city's streets and were cut down by the Mexican defenders. They could not see the men hidden behind walls, loopholes, or rooftops. They tried to march straight down the street until the intense fire drove them to hide in adjacent buildings. Taylor tried to move artillery into the city but it could not hit the well-hidden defenders any better than the U.S. soldiers could. Two days later the US again assaulted the city from two sides and this time they fought differently.
Not wanting to repeat the mistakes of the 21st, General William Jenkins Worth listened to his Texan advisers. These men had fought in Mexican cities before at the Battle of Mier in 1842 and the Battle of Bexar in 1835. They understood that the army needed to "mouse hole" through each house and root out the defenders in close combat.
Worth's men used pick axes to chip holes in the adobe walls of the homes, in the roof of the house from where the soldiers could drop in, or used ladders to climb to the top of a rooftop and assault the Mexican defenders in hand-to-hand combat. The typical assault on a home would include one man who would run to the door of the house and chip the door away with a pick axe under covering fire. Once the door showed signs of weakening, 3–4 other soldiers would run to the door and barge in with revolvers blazing. Worth lost few men on the 23rd using these new urban warfare techniques.