It's about as long as The Civil War (American)
In other words, 3.990 years is 0.999 times the length of The Civil War (American), and the length of The Civil War (American) is 1 times that amount.(a.k.a. War Between the States, a.k.a. the Great Rebellion, a.k.a. the War of the Rebellion) (1861-65) (United States)
The deadliest war in American history, the American Civil War is generally considered to have begun on with the attack on Fort Sumter on April 12th, 1861 and to have lasted until the surrender of Robert E. Lee's army on April 9th, 1865 — 3.990 years. A total of 620,000 American (including Confederate) soldiers are estimated to have lost their lives due to the war.
It's about nine-tenths as long as The First walk around the world
In other words, 3.990 years is 0.9293 times the length of The First walk around the world, and the length of The First walk around the world is 1.08 times that amount.(1970 - 1974) (Dave, John, and Peter Kunst) (including time in recovery)
On June 20th, 1970, Dave Kunst set out from Waseca, Minnesota with $1,000 and a mule. Kunst completed his walk around the world (with a 0.33310 years pause following an assault he suffered during the journey) on October 5th, 1974 — 4.2930 years.
It's about nine-tenths as long as World War I
In other words, 3.990 years is 0.912 times the length of World War I, and the length of World War I is 1.1 times that amount.(a.k.a. First World War, a.k.a. Great War, a.k.a. World War, a.k.a. War to End All Wars, a.k.a. WWI, a.k.a. WW1) (1914 - 1918)
World War I is considered to have begun with the assassination of Austrian Archduke Franz Ferdinand on June 28th, 1914 and to have lasted until the signing of the armistice with Germany on November 11th, 1918, for a total of 4.370 years. However, the Treaty of Versailles which ended conflicts between the major powers was not signed until June 28th, 1919 and the last formal peace treaties were not signed until August 23rd, 1923 — 4.780 years after the armistice was signed.
It's about one-and-three-tenths times as long as The Korean War
In other words, 3.990 years is 1.292 times the length of The Korean War, and the length of The Korean War is 0.774 times that amount.(1951-53) (to armistice signing)
The Korean War between North Korea (a.k.a. Democratic People's Republic of Korea, a.k.a. DPRK) and South Korea (a.k.a. Republic of Korea, a.k.a. ROK) began on June 25th, 1950 during the Cold War. The war paused with the signing of an armistice on July 27th, 1953 — 3.088 years — although the war is officially ongoing and marked by occasional border conflicts.
It's about two-thirds as long as World War II
In other words, 3.990 years is 0.6645 times the length of World War II, and the length of World War II is 1.505 times that amount.(a.k.a. Second World War, a.k.a. WWII, a.k.a. WW2) (1939 - 1945)
The most widespread war in history, the Second World War is generally considered to have started with the German invasion of Poland on September 1st, 1939 and to have until the surrender of Japan on September 2nd, 1945, a total of 6.004 years. Also the deadliest conflict in history, the war had an estimated 60 million casualties globally.
It's about three-fifths as long as The Construction of the Chunnel
In other words, 3.990 years is 0.6246 times the length of The Construction of the Chunnel, and the length of The Construction of the Chunnel is 1.601 times that amount.(a.k.a. Channel Tunnel, a.k.a. Le tunnel sous la Manche) (1987 thru 1994) (Folkstone, Kent, UK to Coquelles, Pas-de-Calais, France)
The construction crew of the Chunnel broke ground in England on December 15th, 1987 and the tunnel was inaugurated by the Queen of England and the President of France on May 6th, 1994 — a total construction time of 6.3820 years. The 4.9 million cubic meters of chalk excavated from the UK side of the Chunnel was dumped within sea walls to create the 299,467.38 sq. km (74 acre) Samphire Hoe County Park near Dover, Kent.
It's about half as long as The Revolutionary War (American)
In other words, 3.990 years is 0.4762 times the length of The Revolutionary War (American), and the length of The Revolutionary War (American) is 2.1 times that amount.(a.k.a. American War of Independence) (1775-83)
The War between Great Britain and the its thirteen colonies in North America (now the United States) for the independence of the United States is generally considered to have started with the Battle of Lexington and Concord on April 18th, 1775 (sometimes known as "The Shot Heard 'round the World") and to have concluded with the signing of the Treaty of Paris on September 3rd, 1783 — 8.3780 years. A total of 25,000 Americans are estimated to have died as a result of the war.