Question: When did Austria invade Serbia in World War I?
About the article "World War I", section "Serbian campaign".

Candidate passages:
Serbia was invaded by Austria-Hungary after Austria-Hungary placed a stringent ultimatum to the Serbian government demanding full compliance to an Austro-Hungarian investigation of complicity by the Serbian government in the assassination of Archduke Francis Ferdinand.
Serbia agreed to most of Austria-Hungary's demands but because it did not fully comply, Austria-Hungary invaded.
Serbia had the diplomatic support of Russia and both Serbia and Russia resented Austria-Hungary's absorption of Bosnia and Herzegovina that held a substantial Serb population, and Serbia had expanded in size through its actions in the Balkan Wars of 1912-1913 when the Ottoman Empire's control in the Balkans collapsed.
During the war, Serbia justified the war as being the result of Austro-Hungarian imperialism towards Serbs and South Slavs, Serbia cooperated with Yugoslavists including the Yugoslav Committee who sought pan-South-Slav unification, particularly through liberating South Slavs from Austria-Hungary.
In the Corfu Declaration in 1917, the Serbian government officially declared its intention to form a state of Yugoslavia.
The first two allied victories in the war were won by Serbian army, on the mountains of Cer and Kolubara, in the western Serbia.
The Austro-Hungarian army was expelled from the country suffering great losses.
Serbia had suffered great losses in the war, losing almost 50% of all men and around 30% of the entire country population.
On July 28, 1918, the Serbian flag was raised at American public buildings, including the White House, on the order of President Woodrow Wilson as a sign of recognition for Serbia's resistance against the Central Powers.  
Montenegro had very close cultural and political connections with Serbia and had cooperated with Serbia in the Balkan Wars of 1912-1913.
Montenegro joined the war against Austria-Hungary. 
Austria-Hungary precipitated the Bosnian crisis of 1908–09 by annexing the former Ottoman territory of Bosnia and Herzegovina, which it had occupied since 1878.
This angered the Kingdom of Serbia and its patron, the Pan-Slavic and Orthodox Russian Empire.
Russian political manoeuvring in the region destabilised peace accords that were already unravelling in what was known as "the powder keg of Europe".
In 1912 and 1913 the First Balkan War was fought between the Balkan League and the fracturing Ottoman Empire.
The resulting Treaty of London further shrank the Ottoman Empire, creating an independent Albanian state while enlarging the territorial holdings of Bulgaria, Serbia, Montenegro, and Greece.
When Bulgaria attacked both Serbia and Greece on 16 June 1913, it lost most of its Macedonian region to Serbia and Greece, the Southern Dobruja region to Romania, and Adrianople (the present-day city of Edirne, which had been conquered by a combined Serbian/Bulgarian force) to Turkey in the 33-day Second Balkan War, which further destabilized the region.
On 28 June 1914, Gavrilo Princip, a Bosnian Serb student and member of multi-ethnic organisation of national revolutionaries called Young Bosnia, assassinated the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand of Austria in Sarajevo, Bosnia.
The political objective of the assassination was and to break the Austro-Hungarian's South-Slav provinces off from the Austro-Hungarian Empire.
The assassination triggered a chain of events that embroiled Russia and the major European powers.
This began a period of diplomatic manoeuvring among Austria-Hungary, Germany, Russia, France, and Britain called the July Crisis.
Wanting to finally end Serbian interference in Bosnia and the Balkans, Austria-Hungary delivered the July Ultimatum to Serbia, a series of ten demands intentionally made unacceptable, intending to provoke a war with Serbia.
When Serbia agreed to only eight of the ten demands, Austria-Hungary declared war on 28 July 1914.
Military historian Hew Strachan argued "Whether an equivocal and early response by Serbia would have made any difference to Austria-Hungary's behaviour must be doubtful.
Franz Ferdinand was not the sort of personality who commanded popularity, and his demise did not cast the empire into deepest mourning".
The dispute between Austria-Hungary and Serbia escalated into what is now known as World War I, which involved Russia, Germany, France, and the United Kingdom.
Within a week, Austria-Hungary had to face a war with Russia, which had the largest army in the world at the time.
The result was that Serbia became just another front to the massive fight that started to unfold along Austria-Hungary's border with Russia.
Serbia had an experienced army, having fought two successful Balkan wars in the previous two years, but it was also exhausted and poorly equipped, which led the Austro-Hungarians to believe that it would fall in less than a month.
Serbia's strategy was to hold on as long as it could and hope the Russians and other Allies could defeat the main Austro-Hungarian Army.
Serbia constantly had to worry about its hostile neighbor to the east, Bulgaria, with which it had fought several wars, most recently in 1913. 
Zichydorf Village was a village that existed for a short period of time near Regina, Saskatchewan, Canada.
The residents of Zichydorf Village mostly came from Zichydorf in Austria-Hungary (now Plandište, Serbia) before World War I. 
Within these nations there were movements to create "greater" nations, that is to enlarge the boundaries of the state beyond those areas where the national ethnic group was in the majority.
This led to conflict between the newly independent nations and the empire from which they split, the Ottoman Empire.
Additionally, it led to differences between the Balkan nations who wished to gain territory at the expense of their neighbors.
Both the conflict with the Ottoman Empire and between the Balkan nations led to the Balkan Wars in 1912-1913.
In a different vein, the ideology of Pan-Slavism in Balkans gained popularity; the movement built around it in the region sought to unite all of the Slavs of the Balkans into one nation, Yugoslavia.
This, however, would require the union of several Balkan states and territory which was part of Austria-Hungary.
For this reason, Pan-Slavism was strongly opposed by Austria-Hungary, while it was supported by Russia which viewed itself as leader of all Slavic nations.
To complicate matters, in the years preceding World War I, there existed a tangle of Great Power alliances, both formal and informal, public and secret.
Following the Napoleonic Wars there had existed a "balance of power" to, in theory, prevent major wars.
This theory held that opposing combinations of powers in Europe would be evenly matched entailing that any general war would be far too costly for any nation to risk entering.
This system began to fall apart as the Ottoman Empire which had been seen as a check on Russian power began to crumble, and as Germany, which had been a loose confederation of minor states, was united into a major power.
Not only did these changes lead to a realignment of power, but of interests as well.
All these factors, and many others, conspired to bring about the First World War.
As is insinuated by the name "the powder keg of Europe," the Balkans were not the major issue at stake in the war, but were merely the catalyst that led to the conflagration.
The Chancellor of Germany in the late 19th century, Otto von Bismarck, correctly predicted it would be the source of major conflict in Europe.
The powder keg "exploded" causing the First World War, which began with a conflict between imperial Austria-Hungary and Pan-Slavic Serbia.
Archduke Franz Ferdinand of Austria, a member of the Austrian royal family, was assassinated by a Bosnian of Serb extraction, a member of the Young Bosnia movement.
Austria-Hungary then issued an ultimatum to Serbia intended to provoke it to war through extreme demands.
The Serbian government reservedly accepted all of the conditions of the ultimatum, except for condition number six which demanded the inclusion of Austria-Hungary in Serbia's judicial inquiry into the assassination.
Serbia indicated this would be unconstitutional and a violation of its sovereignty, and Austria-Hungary used it as a casus belli to invade Serbia.
With Germany fully supporting them, the Austro-Hungarians partially mobilized their army against Serbia, which had already begun mobilizing before replying to the Austro-Hungarian July Ultimatum.
Russia then began mobilizing in support of Serbia, though not bound by treaty to do so.
Due to the system of European alliances, this led to a series of escalating Austrian and Russian mobilizations and eventually Britain and France were also obliged to mobilize and declare war.