For four years, from 1914 to 1918, World War I raged across Europe's western and eastern fronts after growing tensions and then the assassination of Archduke Franz Ferdinand of Austria ignited the war. Trench warfare and the early use of tanks, submarines and airplanes meant the war’s battles were devastatingly bloody, claiming an estimated 40 million military and civilian casualties, including 20 million deaths. Fighting under brutal conditions, World War I battles on both land and at sea saw mass carnage, but few decisive victories, with some conflicts waging for months on end. Below is a timeline of the war's most significant battles.

The first European clash since 1815’s Battle of Waterloo, the Battle of Mons takes place in Mons, Belgium, with a British Expeditionary Force that numbers about 75,000 fighting an estimated 150,000 Germans in an attempt to hold the Mons-Conde Canal. In the final of four “Battles of the Frontier” held in the first weeks of World War I, the British forces are overpowered and forced to retreat, handing the Germans a strategic victory. Some 1,600 British and 5,000 Germans casualties are reported.

Dubbed the Battle of Tannenberg by the victorious Germans in revenge for the 1410 conflict in which the Poles crushed the Teutonic Knights, this would be the country’s biggest win against Russia along the Eastern Front. The battle begins with Russian armies attacking German troops in German East Prussia (now Poland) from the south and the east, which, at first, works. But after intercepting unencrypted radio messages from the Russians, the Germans are able to reorganize their strategy, forcing the Russians into retreat. The Germans pursued the Russians, essentially annihilating the armies with 30,000 casualties and more than 90,000 taken prisoner.

The First Battle of the Marne marks an Allied victory about 30 miles northeast of Paris, where the French army and British Expeditionary Force stop Germany’s swift advance into France. With an exhausted and weakened German force that had sent nearly a dozen divisions to fight in East Prussia and Belgium, the German First Army faces a counterattack and is forced to retreat to the Lower Aisne River, where the first trench warfare of the conflict begins.

In what would become known as the “Race to the Sea,” the First Battle of Ypres begins, the first of three battles to control the ancient Flemish city on Belgium’s north coast that allows access to English Channel ports and the North Sea. The massive conflict—involving an estimated 600,000 Germans and 420,000 Allies—continues for three weeks until brutal winter weather brings it to an end. Typical of so many World War I battles, both sides engage in trench warfare and suffer massive casualties, but neither makes significant gains.

After decoding intercepted German messages, the British Grand Fleet attacks the German Kaiserliche Marine in the North Sea, sparking the Battle of Dogger Bank. The smaller German squadron retreats, but can’t outrun the British. Long-range gunfire ensues but while the German SMS Blücher cruiser is sunk, the British HMS Lion is severely damaged.

The Battle of Verdun becomes World War I’s longest single battle. It lasts nearly a year as the French Army fends off a surprise German offensive that causes mass losses on both sides, with more than 600,000 total casualties.

In an attempt to cripple France’s part in the war and cause a massive blow to its army’s morale, the Germans choose to attack the fort of Verdun, along the banks of the Meuse River. The Germans make advances in the bloody conflict until July when their offensive is called off. The French then begin retaking the stronghold and, as winter sets in and the first Battle of the Somme rages, the Verdun fighting finally comes to an end.

In modern warfare’s first major beach landing, the Gallipoli Campaign sees British and French troops invading the Ottoman Empire at the peninsula of Gallipoli in the Dardanelles Straits (now western Turkey). The invasion is an effort to control the sea route and seize Constantinople. With Western Front fighting stalled, the Ally forces intend the attack to be a swift victory, but ultimately withdraw, suffering some 180,000 casualties, including more than 28,000 Australian soldiers.

World War I’s biggest naval conflict, the Battle of Jutland off the coast of Denmark marks the first and only showdown between German and British battleships. After German forces attack the Royal Navy, 250 ships and 100,000 men take part in the bloody fight, with both sides losing thousands of lives and several ships. Although there is no clear victor, Britain is able to secure North Sea shipping lanes and continue a blockade of German ports. This blockade proves pivotal to the Allies eventually winning the war.

During one of history's bloodiest battles, on the first day alone of the first Battle of the Somme, British forces suffer more than 57,000 casualties, including 20,000 deaths, as they attempt to overrun German trenches and are easily gunned down.

The Allies soon change tactics in their attempt to fight back the Germans on the Western Front along the Somme River in France, but make minimal breakthroughs over a nearly five-month period. Notable for the firsts use of tanks, the battle finally ends with more than a million casualties.

The 12 battles held along the Italian Front at the Isonzo River at the Adriatic Sea (now part of Slovenia), see the Italians repeatedly attacking the Austrians to gain control of the area and entry to Vienna. After Italy makes some progress after multiple failed attempts, Germany eventually joins the Austrian troops, forcing Italy into retreat.

Also known as the Third Battle of Ypres, the Battle of Passchendaele takes place in Ypres, Belgium, as British forces, with help from the French and the use of tanks, launch an attack to wrest control of Ypres from the Germans. Attacks and counterattacks ensue for four months in the rain and mud, with Canadian forces brought in to help relieve the troops but little ground being won. In the end, it is considered a victory for the Allies, with but one that costs both sides more than 550,000 casualties.

In its first attack as a unified force, the Canadian Corps, consisting of the four Canadian divisions, launches an Easter Sunday offensive at Vimy Ridge in northern France, claiming a quick and decisive victory over the Germans in three days. Part of the Allied Battle of Arras, the well-planned battle uses new artillery tactics and marks the corps as an elite force.

In an attack by Russian forces against the Austro-Hungarians and Germans in Galicia, the June Offensive (also called the Kerensky Offensive and the July Offensive) operation takes place, ordered by Russian Minister of War Alexander Kerensky against the nation's popular calls for peace. Despite early gains, Russian troops suffer mass casualties and soon revolt. They are quickly overtaken by an Austro-German counterattack and the Russian army essentially disintegrates.

Immortalized by Ernest Hemingway's A Farewell to Arms, the Battle of Caporetto, also called the 12th Battle of the Isonzo, is waged on the Italian Front near Kobarid (now part of Slovenia). German and Austro-Hungarian forces soundly defeat the Italian front line, resulting in nearly 700,000 Italian casualties and seriously diminishing morale.

In World War I's first large-scale tank offensive, the Battle of Cambrai near Cambrai, France, ultimately gains little ground, but changes the course of modern warfare with the use of tank brigades and new artillery methods.

On November 20, British forces engage in a surprise attack, gaining some new territory over the next several days. But on November 30, a massive German counterattack results in most of that ground being recovered.

Fought along the Somme River basin in France, the Second Battle of the Somme is launched by the Germans, hoping to capitalize on the Russian army's collapse and attacking British trenches with gas and artillery fire. The British are forced into retreat and the Germans win their biggest single territorial gain along the Western Front since the war's onset. But within a week the Allies regroup and the German offensive begins to lose steam and is eventually halted.

Also known as the Ludendorff Offensive, the 1918 Spring Offensive begins with the Germans launching a string of attacks along the Western Front in hopes of winning the war before U.S. troops can join the Allies. Despite making successful advances in four attacks, the territory they retake or newly control doesn’t lead to strategic gains. With the American forces arriving in July, a counteroffensive and exhausted soldiers, the Germans, while claiming victory, are badly weakened.

In their last offensive attack of the war, the Germans strike Ally troops near the Marne River in France's Champagne region in a diversionary attempt to lure them from a separate planned attack in Flanders. But fooled by a set of false trenches implemented by the French, the Germans are met by heavy fire and a counterattack by French and American troops as they approach the actual front lines and are forced to retreat.

The opening attack of what would be come to be called the Hundred Days Offensive, the Battle of Amiens sees one of the most successful advances of World War I, with Allied troops securing more than eight miles in the conflict’s first fog-covered day, later called "the black day of the German Army" by General Erich Ludendorff. Catching the Germans by surprise, the Allies attack with the help of 2,000 guns, 1,900 planes and 500 tanks, causing large-scale German casualties and a fatal blow to morale.

More than 1 million American soldiers take part in the Battles of the Meuse-Argonne in France's dense Forest of Argonne and along the Meuse River, making it the American Expeditionary Forces' biggest World War I operation. It would leave 26,000 Americans dead, with 120,000-plus casualties—the deadliest battle in U.S. history. Joined by the French and aided by tanks and U.S. Air Service planes, the Allies capture tens of thousands of German prisoners and, after four months, Germany finally cedes, beginning its last retreat.

As part of the Hundred Days Offensive, British and Canadian Corps forces strike a decisive victory in Cambrai in northern France, which had been held by Germany since 1914. Surrounded, exhausted and with disintegrating morale, the Germans face the certainty that the war has been lost.

Fought on World War I's final day, the Canadian Corps captures Mons, Belgium, held by the Germans since 1914, in the Battle of Mons. The early morning offensive happens hours before troops learn that Germany has agreed to an armistice at 11 a.m. It also marks the final death of an Allied soldier, a Canadian shot by a sniper minutes before the gunfire ends.