William II (Dutch: Willem II, French: Guillaume II; 6 December 1792 – 17 March 1849) was King of the Netherlands, Grand Duke of Luxembourg, and Duke of Limburg. He reigned for nearly nine years, making him the shortest-reigning monarch in Dutch history.

William II was the son of William I and Wilhelmine of Prussia. When his father, who up to that time ruled as sovereign prince, proclaimed himself king in 1815, he became Prince of Orange as heir apparent of the United Kingdom of the Netherlands. With the abdication of his father on 7 October 1840, William II became king. During his reign, the Netherlands became a parliamentary democracy with the new constitution of 1848. William II was married to Anna Pavlovna of Russia, with whom he had five children. William II was succeeded by his son William III.

Willem Frederik George Lodewijk was born on 6 December 1792 in The Hague. He was the eldest son of the then Prince William and Wilhelmine of Prussia. His maternal grandparents were King Frederick William II of Prussia and his second wife Frederika Louisa of Hesse-Darmstadt. William had one younger brother, Frederick, and two younger sisters, Pauline and Marianne.

When William was two, he and his family fled to England after allied British-Hanoverian troops left the republic and entering French troops defeated the army of the United Provinces, claiming liberation by joining the anti-Orangist Patriots. William spent his youth in Berlin at the Prussian court, where he followed a military education and served in the Prussian Army. After this, he studied civil law at Christ Church, University of Oxford.

He entered the British Army, and in 1811, as a 19-year-old aide-de-camp in the headquarters of Arthur Wellesley, 1st Duke of Wellington, was allowed to observe several of Wellington's campaigns of the Peninsular War. Though not yet 20, the young prince, according to the customs of the time, was made lieutenant colonel on 11 June 1811 and colonel on 21 October that year. He took part in the Siege of Badajoz and the Battle of Salamanca in Spain in 1812. Later that year, on 8 September, he was made an aide-de-camp to the Prince Regent and on 14 December 1813 promoted to major-general. His courage and good nature made him very popular with the British, who nicknamed him "Slender Billy". He returned to the Netherlands in 1813 when his father became sovereign prince, and in May 1814 succeeded Sir Thomas Graham as the highest-ranking officer of the British forces stationed there.

On 8 July 1814, he was promoted to lieutenant-general in the British Army, and on 25 July to general. As such, he was senior officer of the Allied army in the Low Countries when Napoleon I of France escaped from Elba in 1815. He relinquished command on the arrival of the Duke of Wellington, and, though this was his first real battle, served as commander of the I Allied Corps, first at the Battle of Quatre Bras (16 June 1815) and then at the Battle of Waterloo (18 June 1815), where he was wounded in his left shoulder by a musket ball. He was aged 22. As a sign of gratitude for what the Dutch throne styled "his" victory at Waterloo, William was offered Soestdijk Palace by the Dutch people.

Military historian William Siborne blamed many casualties suffered by Coalition forces during the Waterloo campaign to William's inexperience. In response, Siborne was accused by Lieutenant-General Willem Jan Knoop of misrepresenting William's actions at Waterloo. An inspection into the archives of Siborne by Dutch officer Francois de Bas in 1897 claimed to discover the "selective use of sources" and "numerous miscounts and untruths".

In 1814, William was briefly engaged to Princess Charlotte of Wales, only child of the Prince Regent (later George IV of the United Kingdom) and his estranged wife, Caroline of Brunswick. The engagement was arranged by the Prince Regent, but it was broken off because Charlotte's mother was against the marriage and because Charlotte did not want to move to the Netherlands.^{[citation needed]}