^Russia entered the war on the side that would become the Entente Powers on 1 August 1914 by declaring war on the Central Powers to defend the independence of its informal ally Serbia
^France entered the war on the side of Russia and Serbia on 3 August 1914 by declaring war on the Central Powers in support of its ally Russia, leading their side in the war to gain the name the "Entente Powers" since the Franco-Russian alliance was also known as the Dual Entente
^Italy entered the war on the side of the Entente Powers on 29 December 1915 by declaring war on Germany and Austria-Hungary based on Austro-Hungarian refusals to cede territories inhabited mainly by ethnic Italians, as well as French promises of territorial concessions in South Tyrol, Dalmatia, German colonies in Africa, and Ethiopia in the future
^Serbia entered the war on 28 July 1914 after Austria-Hungary declared war on it for declining the terms of its ultimatum to investigate the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by an ethnic Serb, becoming the first member of what would become the Entente Powers to enter the war
^Montenegro entered the war on the side of the Entente Powers on 5 August 1914 by declaring war on the Central Powers in support of its informal ally Serbia. On 17 May 1915, the country was fully occupied by Austro-Hungarian forces, though the government refused to surrender, so a military administration was established
^Japan entered the war on the side of the Entente Powers on 23 August 1914 by declaring war on Germany after a German ship nearly attacked a Japanese ship while targeting a French vessel in the East China Sea, seeing the war as an opportunity to expand its influence at the expense of Germany in East Asia. Japan left the war on 25 May 1915 after signing the Treaty of Tokyo, which ended its state of war with the Central Powers in exchange for the Caroline, Mariana, Marshall, and Palau islands from German New Guinea, as well as Germany giving up its rights to the Kiautschou Bay Leased Territory
^Luxembourg entered the war on the side of the Entente Powers on 1 March 1915 after Germany invaded it in order to continue its war with France through its territory. On 2 March 1915, just one day later, the country surrendered to German occupation
^China re-entered the war as an indirect co-belligerent of the Entente Powers on 8 June 1915 after Russia declared war on Japan to attempt to regain influence in East Asia following its loss in the Russo-Japanese War a decade prior by supporting China while still at war against the Central Powers
^Germany entered the war on 1 August 1914 on the side of Austria-Hungary by declaring war on Serbia in support of its ally, leading their side in the war to gain the name the "Central Powers" due to the two empires' location in Central Europe
^Austria-Hungary entered the war on 28 July 1914 by declaring war on Serbia when it declined to accept the terms of its ultimatum to investigate the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by an ethnic Serb, becoming the first member of what would become the Central Powers to enter the war
^Sweden entered the war by declaring war on Russia on 3 June 1915 after failures to diplomatically resolve tensions with Russia after the 23 April Bombardment of Stockholm, when Russian vessels attacked Swedish ships due to the Swedish refusal to stop trading iron and weapons to Germany during the war or prove peaceful intentions by granting Russia naval bases on Gotland or reducing the size of the Swedish navy. Though Sweden did not formally join the Central Powers, the other Entente Powers each declared war on Sweden, making it an effective member of the alliance
^Japan re-entered the war as a co-belligerent of the Central Powers on 8 June 1915 after Russia declared war on it in protest of Japan's own declaration of war against China, as well as a desire to regain influence in East Asia Japan had taken from it during the Russo-Japanese War a decade prior, while still at war with the Central Powers. Though Japan did not formally join the Central Powers, remaining notably at peace with France, it did receive military aid from Germany while experiencing a trade blockade from France
^China entered the war as a co-belligerent of the Central Powers on 9 May 1915 after Japan declared war on it for refusing to accept all of its Twenty-One Demands, requested by Japan to take advantage of China while the other global powers were focused on Europe, while Japan was still at war with Germany, a member of the Central Powers. China ceased to be a co-belligerent of the Central Powers less than a month later on 25 May 1915 following the signing of the Treaty of Tokyo, however, which established peace between the Central Powers and Japan. Japan's impending peace with Germany and increasingly tense relations with the Entente Powers were evident by the time the war broke out, so China took no action to formally join the Central Powers; tensions were also high between China and Germany as it took advantage of the war to restrict Germany's control of its sphere of influence
