France an England got involved because they declared war onto Germany when Germany invaded Poland. The USSR got involved fully when Germany invade the USSR in 1941. The US got involved after PearlHarbor. Other countries got involved because theydidn'twant to beoccupiedby the Axis or the Allies.
I think it is because Germany wanted war.
Axis Power's conquest of other countries.
Which war? Probably because so many countries were involved and waging war at the same time for various and/or related reasons.
Some countries have accurace (or as accurate as possible) records of who fought in their forces in World War I; but some countries for various reasons have little or no records of this information.
Most countries go to war for two reasons. The two reasons why countries go to war are because of power and money.
The US was across an ocean from the European countries, which was a major reason why they decided to stay out of war.
The USA joined World War I after the Germans sank the Lusitania, a British cruise-liner, containing hundreds of Americans. Their entrance is similar to Belgium's, as they were attacked by Germany. Other countries entered the war for reasons of alliances or reasons of wanting to gain land.
Japan was intent on "ruling the world" and they wanted the resources the Asian countries had, such as metal ores, oil, food, and people to fight their war.
USA and USSR
The United States participated in the First World War because Germany had capsized US trade ship. The European countries wanted to stop German expansion.
The reasons of the outbreak of the 1st world war are:- 1.due to the industrial revolution the European countries started fighting with each other to acquire colonies in Asia and Africa. 2.The European countries wanted more raw materials in large scale. 3.They needed markets to sell their goods.
Chicken and waffles.