The intensive construction of Belgrade began when it was permanently liberated from the Turks and this continued into the 20th century. With the construction of the train station and the port on the Sava, the focus of the city shifted and the former Turkish-Oriental area, known as Dorćol, began to lose its importance. However, Belgrade's further development was impeded due its position as a border town facing Austria - as such it posed an obstacle to Austria's goals of advancing into the Balkans.

A reason was needed to justify Germany's and Austria's Balkan expansion plans. When the Austro-Hungarian heir to the throne, Franz Ferdinand, was assassinated in Sarajevo on 28th June, 1914, such a reason presented itself. The First World War started with an ultimatum and then an attack on Serbia. On 2nd December, 1914, during a four-month bombing campaign, the Austrian advance guard entered Belgrade, but retreated on 15th December. The occupying Austrians had to retreat, as Serbia had strengthened its position after victory in the Battle of Kolubara. Led by Field Marshal Mackensen, a second attack was launched on the 6th and 7th of October 1915 and lasted a full seven days and nights. In spite of 9731 injured and dead soldiers, the Austrian army managed to enter the city and Belgrade suffered for three years under Austrian occupation. After breaching the Macedonian (Salonika) Front, the Serbian Army and units of the Allied Forces liberated Belgrade on November 1st, 1918. Serbia lost 28% of its population in World War One and Belgrade was one of the most severely affected cities.

Soon after liberation Belgrade became the capital of the newly proclaimed Kingdom of Serbs, Croats and Slovenes and the city once again began to flourish. With Zemun firmly within city limits, Belgrade was no longer a border town and was free to quickly develop. In addition to the town's rapid growth out towards Avala, Košutnjak, Čukarica and the Danube, the heart of the old town saw a great deal of construction, which gave Belgrade a more European feel. However, after the assassination of King Aleksandar I Karađorđević, the forces that showed sympathy towards the new world order of Hitler and Mussolini began to grow in strength and the country signed the Tripartite Pact on March 25th, 1941. This led to mass demonstrations and a coup d'etat two days later, which successfully brought down the government.

Only a few days later, Belgrade was targeted in a dreadful bombing campaign. During the assault on the city, carried out by the German Luftwaffe on 6th and 7th April 1941, 2274 people were killed and the number of injured was several times greater. Several thousand buildings were damaged to a greater or lesser extent, including the cultural and historical landmark, the National Library (Narodna biblioteka), which was completely destroyed by fire. This was just the introduction to the four year occupation which followed the bombing. German forces entered Belgrade without resistance on April 12th, 1941. In addition to the suffering and persecution inflicted on the city, Belgrade's citizens were also victim to Allied bombing campaigns, particularly in the spring and autumn of 1944. Many buildings were damaged, including residential buildings, and all the bridges spanning the Sava and the Danube were destroyed. Around 1160 citizens lost their lives in these campaigns. Approximately 50,000 people were killed during the Second World War in Belgrade, and the city suffered incalculable damage. On 20th October, 1944, units of the Yugoslav Liberation Army and the Russian Red Army moved into the city and liberated Belgrade.

With a new communist government in power, political opponents were arrested and liquidated and Belgrade's youth was mobilised and sent to the Srem Front. In spite of extensive destruction and general poverty, Belgrade slowly began to recover from the devastating consequences of war and occupation. The economic infrastructure destroyed during the war was rebuilt and new facilities constructed, particularly industrial plants (primarily focusing on the metal, chemical and power industries), the transport infrastructure was developed and cultural and education institutions were restored and expanded. On 29th November, 1945, a declaration was passed abolishing the monarchy, and the Federal People's Republic of Yugoslavia - the FPRY (FNRJ) was proclaimed as a federal state with a republican political system, which marked the official beginning of Josip Broz Tito's communist government. The constitution of the FPRY was adopted on 31st Janurary, 1946, and so a socialist state union of the South Slav peoples was born and the nationalisation of the properties of pre-war industrialists began.

Due to the characteristic policies implemented by Josip Broz Tito, balancing Yugoslavia between East and West, Belgrade developed into an important political, cultural, economic and sporting centre. Important international meetings were held in Yugoslavia, including the First Conference of the Non-Aligned Countries in 1961 and the Conference on Security and Co-operation in Europe (CSCE), as well as many cultural and sporting events.

Student unrest in 1968 directed against unjust social differences and bureaucratic inflexibility, revealed cracks in the socialist system. In 1974, changes were made to the Constitution, giving the republics within Yugoslavia greater rights and the state moved towards confederation. Kosovo and Metohija and Vojvodina were autonomous provinces within the republic of Serbia, but in essence they functioned like independent republics in their own right. With the death of Josip Broz Tito on May 4, 1980, the Yugoslav disaster began, first economically and then politically. The disintegration of Yugoslavia began in 1991 as unresolved national and ethnic problems flared up and civil war broke out, bringing to an end the longest period of peace in the Balkans in the 20th century. In June 1991, Slovenia and Croatia declared independence from Yugoslavia, closely followed by Bosnia and Herzegovina and Macedonia in 1992. Hence in 1992 Belgrade became the capital of the Federal Republic of Yugoslavia, which consisted solely of the Republic of Serbia and the Republic of Montenegro. Reacting to the war in Bosnia and Herzegovina, the United Nations Security Council imposed economic sanctions against FR Yugoslavia in 1992. In 1993, as a result of wars, sanctions, economic collapse, hundreds of thousands of refugees and an irresponsible government, Yugoslavia experienced the highest rate of inflation in history.

A series of civil wars began in 1991, starting off with a short-lived conflict between the Yugoslav National Army (JNA) and Slovenia. The war spread to Croatia (1991-1995) when Serbs, reacting to their loss as a constituent people under the new Croatian constitution, started a rebellion and Croats sought to create a nationally homogenous state. The Croatian war came to an end when the Croatian army took action against the Serbian population and expelled over 300,000 Serbs from the territory. The civil war in Bosnia and Herzegovina (1992-1995) was an ethnic and religious war between Serbs, Croats and Muslims, which was ended by the signing of the Dayton Agreement. Between 1996 and 1999, there was conflict in Kosovo and Metohija between the Kosovo Liberation Army (a banned armed separatist organisation of Kosovo Albanians) and the Serbian military and police forces. After unsuccessful negotiations in Rambouillet, NATO began its bombing campaign against FR Yugoslavia, which began on March 24th and continued until June 10th, 1999.

In February 2003, the state union of Serbia and Montenegro was formed. In May 2006, Montenegro declared independence from Belgrade, which then became the capital of a new, independent Serbian state. The new constitution of the Republic of Serbia was passed in November 2006.