Why Did Other European Powers Join the Austria-Hungary vs. Serbia Conflict (WWI)?

The July Crisis of 1914 escalated from a Balkan quarrel into a world war through a cascade of rigid alliance commitments, strategic imperatives, and diplomatic miscalculations:

1. Alliance Chains: Austria-Hungary's ultimatum to Serbia triggered Russia's mobilization (as Slavic protector). Germany, bound to Austria by the Dual Alliance, mobilized against Russia and its ally France. Germany's Schlieffen Plan required invading Belgium to knock out France quickly, violating Belgian neutrality guaranteed by Britain—drawing in the UK.
2. Balance of Power: No power wanted a hegemonic Germany or a collapsed Austria-Hungary. Britain feared German Channel ports; France sought to regain Alsace-Lorraine; Russia aimed for Constantinople/Straits.
3. Nationalism & Imperialism: Balkan nationalism (Serbian irredentism) destabilized the multi-ethnic Austro-Hungarian Empire. Imperial competition for colonies and naval supremacy (Anglo-German race) created zero-sum tensions.
4. Military Timetables: Mobilization was considered irreversible and offensive. Russia's partial mobilization became general; Germany's plan demanded immediate attack on France. Diplomacy had no time to work.
5. Domestic Politics: Leaders feared appearing weak; some (e.g., German military) saw war as preferable to perceived encirclement. Public opinion, inflamed by press, often supported "defensive" war.

The system had no circuit-breaker. A localized conflict became general because the great powers' security doctrines were interlocked and offensive.