The Acts of Union 1800 were a series of legislative measures enacted by the Parliament of Great Britain and the Parliament of Ireland, which led to the merging of the Kingdom of Great Britain and the Kingdom of Ireland into a single political entity known as the United Kingdom of Great Britain and Ireland. The Union came into effect on January 1, 1801, following its passage in both parliaments. The driving forces behind the union were multifaceted, including the political instability in Ireland, the desire for greater security against perceived foreign threats, and the need for a more cohesive economic strategy amidst the challenges of the Napoleonic Wars.

The union was born out of an extensive negotiation process characterized by debates and compromises. Key figures involved included British Prime Minister William Pitt the Younger, who championed the union as a means to strengthen Britain politically, and Irish leaders who saw the potential for expanded representation and economic benefits. The union was largely motivated by a desire to stabilize Ireland's governance and increase its representation in the British Parliament, which was seen as crucial during a period of upheaval.

Despite the intended benefits, the Union was met with significant resistance in Ireland, where many viewed it as an imposition. The merger reduced the Irish Parliament's power, consolidating legislative authority in Westminster, and sowed seeds for future discontent. Furthermore, the union had long-lasting implications for Irish identity and political relations, as it ultimately influenced the nationalist movements throughout the 19th and 20th centuries.