Reichskommissariat countries (and even countries that have the same flag as the German Reich) will not be present in this list
Kingdom of Hungary
Magyar Királyság
Sign the Tripartite Pact on: 20 November, 1940
Kingdom of Romania
Regatul României
Sign the Tripartite Pact on: 23 November, 1940
Tsardom of Bulgaria
Царство България
Sign the Tripartite Pact on: 1 March, 1941
Kingdom of Yugoslavia
Краљевина Југославија
Kraljevina Jugoslavija
Sign the Tripartite Pact on: 25 March, 1941
Hashemite Kingdom of Iraq
المملكة العراقية الهاشمية
Invaded by United Kingdom of Great Britain and Northern Ireland on: 2 May, 1941
Japanese Empire
大日本帝國
Joined the war on: 8 December, 1941
Kingdom of Thailand
ราชอาณาจักรไทย
Declared war at United Kingdom of Great Britain and Northern Ireland on: 25 January, 1942
Puppet States:
Slovak Republic
Slovenská republika
Joined the war on: 1 September, 1939 - Puppet State of German Reich
Protectorate of Bohemia and Moravia
Protektorát Čechy a Morava
Protektorat Böhmen und Mähren
Joined the war on: 1 September, 1939 - Puppet State of German Reich
General Government
Generalne Gubernatorstwo
Generalgouvernement
Joined the war on: 26 October, 1939 - Puppet State of German Reich
Kingdom of Albania
Mbretënia e Shqipërisë
Regno d'Albania
Joined the war on: 10 June, 1940 - Puppet State of Kingdom of Italy
Independent State of Croatia
Nezavisna Država Hrvatska
Joined the war on: 10 April, 1941 - Puppet State of German Reich
Hellenic State
Ἑλληνική Πολιτεία
Joined the war on: 30 April, 1941 - Puppet State of German Reich and Kingdom of Italy
Government of National Salvation
Влада народног спаса
Joined the war on: 29 August, 1941 - Puppet State of German Reich
Governorate of Montenegro
Governatorato del Montenegro
Joined the war on: October, 1941 - Puppet State of Kingdom of Italy
Empire of (Great) Manchuria
(大)滿洲帝國
Joined the war on: 8 December, 1941 - Puppet State of Japanese Empire
Mengjiang United Autonomous Government
蒙疆聯合自治政府
Joined the war on: 8 December, 1941 - Puppet State of Japanese Empire
Reorganized Nationalist Government of the Republic of China
中華民國國民政府改組
Joined the war on: 8 December, 1941 - Puppet State of Japanese Empire
Kingdom of Kampuchea
ព្រះរាជាណាចក្រកម្ពុជា
カンボジア王国
Joined the war on: 8 December, 1941 - Puppet State of Japanese Empire
French Protectorate of Laos
ລາວ
Joined the war on: 8 December, 1941 - Puppet State of Japanese Empire and a colony of French State
the National Government
Den nasjonale regjering
Joined the war on: 1 February, 1942 - Puppet State of German Reich
State of Monaco
État de Monaco
Joined the war on: 11 November, 1942 - Puppet State of Kingdom of Italy
German-occupied territory of Montenegro
Deutsch besetztes Gebiet von Montenegro
Joined the war on: 7 September, 1943 - Puppet State of German Reich
Albanian Kingdom
Mbretëria Shqiptare
Joined the war on: 8 September, 1943 - Puppet State of German Reich
Italian Social Republic
Repubblica Sociale Italiana
Joined the war on: 23 September, 1943 - Puppet State of German Reich
Republic of the Philippines
Repúbliká ng Pilipinas
República de Filipinas
フィリピン共和国
Joined the war on: 14 October, 1943 - Puppet State of Japanese Empire
Provisional Government of Free India
आर्ज़ी हुकूमत-ए-आज़ाद हिन्द
Joined the war on: 21 October, 1943 - Puppet State of Japanese Empire
Belarusian Central Council
Weißruthenischer Zentralrat
Беларуская цэнтральная рада
Joined the war on: 21 December, 1943 - Puppet State of German Reich
Independent State of Macedonia
Независима държава Македония
Joined the war on: 8 September, 1944 - Puppet State of German Reich
Government of National Unity
Nemzeti Összefogás Kormánya
Joined the war on: 17 October, 1944 - Puppet State of German Reich
Kingdom of Cambodia
ព្រះរាជាណាចក្រកម្ពុជា
カンボジア王国
Joined the war on: 9 March, 1945 - Puppet State of Japanese Empire
Empire of Vietnam
Đế quốc Việt Nam
越南帝國
Joined the war on: 9 March, 1945 - Puppet State of Japanese Empire
Kingdom of Luang Phrabang
ອານາຈັກຫຼວງພະບາງ
ルアンプラバン王国
Joined the war on: 8 April, 1945 - Puppet State of Japanese Empire
Questionable Belligerents:
Spanish State
Estado Español
Send troops to help the Axis (Blue Division) on: 24 June, 1941
Spanish Protectorate in Morocco
Protectorado español en Marruecos
الحماية الإسبانية على المغرب
Protectorate of Spanish State - 24 June, 1941
Portuguese Republic
República Portuguesa
Send 150 troops to help the Axis (Blue Division) on: 24 June, 1941
Kingdom of Denmark
Kongeriget Danmark
Start supporting the Axis on: 29 June, 1941
Republic of Finland
Suomen tasavalta
Invaded the Soviet Union together with the Axis on: 25 June, 1941
Japanese Imperial Army
大日本帝國陸軍
Japanese Empire's surrender: 2 September, 1945 (- These are troops that continued fightining even after Japanese Empire's surrender)
See about the Second Sino-Japanese War for more informations