The Nine Years' War was a European great power conflict from 1688 to 1697 between France and the Grand Alliance. Fought primarily in Europe, related conflicts include the Williamite war in Ireland, and King William's War in North America.

The 1678 Treaty of Nijmegen that ended the Franco-Dutch War was the highpoint of the French expansionist policies pursued by Louis XIV. Over the next few years, he continued attempts to strengthen France's frontiers, culminating in the 1683 to 1684 War of the Reunions. The Truce of Ratisbon guaranteed these new borders for twenty years, but concerns among European Protestant states over French expansion and anti-Protestant policies led to the creation of the Grand Alliance, headed by William of Orange.

In September 1688 Louis led an army across the Rhine to seize additional territories beyond it. This move was designed to extend his influence and pressure the Holy Roman Empire into accepting his territorial and dynastic claims. However, Leopold I, Holy Roman Emperor and German princes supported the Dutch in opposing French aims, while the November 1688 Glorious Revolution secured English resources and support for the Alliance. Over the next few years, fighting focused around the Spanish Netherlands, the Rhineland, the Duchy of Savoy, and Catalonia. Although engagements generally favoured Louis' armies, neither side was able to gain a significant advantage, and by 1696 the main belligerents were financially exhausted, making them keen to negotiate a settlement.

Under the terms of the 1697 Peace of Ryswick, French control over the entirety of Alsace was officially recognized, but Lorraine and gains on the right bank of the Rhine were relinquished and restored to their rulers. Louis XIV also recognised William III as the rightful king of England, while the Dutch acquired barrier fortresses in the Spanish Netherlands to help secure their borders and were granted a favorable commercial treaty, while Spain regained Luxembourg and other territories from France. However, both sides viewed the peace as only a pause in hostilities, since it failed to resolve who would succeed the ailing and childless Charles II of Spain as ruler of the Spanish Empire, a question that had dominated European politics for over 30 years. This would lead to the War of the Spanish Succession in 1701.

In the years following the Franco-Dutch War (1672–1678), Louis XIV of France, now at the height of his power, sought to impose religious unity in France and to solidify and expand his frontiers. He had already won personal glory by conquering new territory, but he was no longer willing to pursue an open-ended militarist policy of the kind that he had undertaken in 1672. Instead, he would rely upon France's clear military superiority to achieve specific strategic objectives along his borders. Proclaimed the "Sun King", a more mature Louis, conscious that he had failed to achieve decisive results against the Dutch, had turned from conquest to security by using threats, rather than open war, to intimidate his neighbours into submission.

Louis XIV, along with his chief military advisor, Louvois, his foreign minister, Colbert de Croissy, and his technical expert, Vauban, developed France's defensive strategy. Vauban had advocated a system of impregnable fortresses along the frontier to keep France's enemies out. To construct a proper system, however, the King needed to acquire more land from his neighbours to form a solid forward line. That rationalisation of the frontier would make it far more defensible and define it more clearly in a political sense, but it also created the paradox that while Louis's ultimate goals were defensive, he pursued them by offensive means. He grabbed the necessary territory in the Reunions, a strategy that combined legalism, arrogance and aggression.

The Treaties of Nijmegen (1678) and the earlier Peace of Westphalia (1648) provided Louis XIV with the justification for the Reunions. These treaties had awarded France territorial gains, but owing to the vagaries of their language (as with most treaties of the time) they were notoriously imprecise and self-contradictory, and never specified exact boundary lines. That imprecision often led to differing interpretations of the text and resulted in long disputes over frontier zones, where one side might gain a town or area and its "dependencies", but it was often unclear what the dependencies were. The machinery needed to determine the territorial ambiguities was already in place through the medium of the parlements at Metz (technically, the only Chamber of Reunion), Besançon and a superior court at Breisach, dealing respectively with Lorraine, Franche-Comté and Alsace. The courts usually found in Louis XIV's favour. By 1680, the disputed County of Montbéliard, lying between Franche-Comté and Alsace, had been separated from the Duchy of Württemberg, and by August, Louis XIV had secured the whole of Alsace with the exception of Strasbourg. The Chamber of Reunion of Metz soon laid claims to land around the Three Bishoprics of Metz, Toul and Verdun and most of the Spanish Duchy of Luxembourg. The fortress of Luxembourg City itself was then blockaded with the intention of it becoming part of his defensible frontier.

On 30 September 1681, French troops also seized Strasbourg and its outpost, Kehl, on the right bank of the Rhine, a bridge that Holy Roman Empire ("imperial") troops had regularly exploited during the latter stages of the Dutch War. By forcibly taking the imperial city, the French now controlled two of the three bridgeheads over the Rhine, the others being Breisach, which was already in French hands, and Philippsburg, which Louis XIV had lost by the Treaty of Nijmegen. On the same day that Strasbourg fell, French forces marched into Casale, in northern Italy. The fortress was not taken in the process of the Reunions but had been purchased from the Duke of Mantua, which, together with the French possession of Pinerolo, enabled France to tie down Victor Amadeus II, the Duke of Savoy, and to threaten the Spanish Duchy of Milan. All of the Reunion claims and annexations were important strategic points of entry and exit between France and its neighbours and were immediately fortified by Vauban and incorporated into his fortress system.