Austria didn't declare war against Serbs (the ethnic group), but it declared war and invaded Serbia (the country in which only a part of the ethnic group lived). Moreover, Gavrilo Princip wasn't even a citizen of Serbia but the citizen of Austria-Hungary, and the assassination wasn't sponsored by Serbia either.
An interesting data is that the invading Austrian army consisted in large part of ethnic Serbs from Bosnia that were serving in Austrian army.
Yes, Gavrilo Princip was a Serbian by ethnicity, like almost half of the population of Bosnia at the time, a then recently annexed province that was previously occupied by Ottoman Empire (Turkey). In fact, there probably was few ethnic Austrians in Bosnia, since Austria-Hungary was an Empire that simply treated Bosnia and many other similar regions as colonies. So, Gavrilo Princip was a member of an underground indigenous movement who was fighting the colonial power. It is debatable whether it is terrorism or not (one side's villains are the other side's heroes), but it is a historical fact that Serbia (the country) didn't instruct him, nor helped him, nor the assassination was in Serbia's interest. Simply put, the Austria was eyeing to invade Serbia for a long time before that, and used the assassination as a pretext for invasion that was going to happen sooner or later no matter what. Serbia accepted many outrageous Austrian demands, but Austria still invaded...
An interesting data is that the invading Austrian army consisted in large part of ethnic Serbs from Bosnia that were serving in Austrian army.
Yes, Gavrilo Princip was a Serbian by ethnicity, like almost half of the population of Bosnia at the time, a then recently annexed province that was previously occupied by Ottoman Empire (Turkey). In fact, there probably was few ethnic Austrians in Bosnia, since Austria-Hungary was an Empire that simply treated Bosnia and many other similar regions as colonies. So, Gavrilo Princip was a member of an underground indigenous movement who was fighting the colonial power. It is debatable whether it is terrorism or not (one side's villains are the other side's heroes), but it is a historical fact that Serbia (the country) didn't instruct him, nor helped him, nor the assassination was in Serbia's interest. Simply put, the Austria was eyeing to invade Serbia for a long time before that, and used the assassination as a pretext for invasion that was going to happen sooner or later no matter what. Serbia accepted many outrageous Austrian demands, but Austria still invaded...