Have to do each one separately I think, pinkquartz Like I just did
I think that it was all territorial and vying for power in Europe, the middle East and India. The French and the Prussians had been at each other's throats for years, exacerbated by Bismark 'unifying' Germany by taking control of independent German states. Britain had made various defensive alliances with other European nations, including Belgium, to fend off Prussian aggression and expansionist ambitions. When Prussia invaded countries the British were allied to we were obliged to send troops to their defence.
I think it was a bit of family rivalry, too. Kaiser William greatly disliked his cousin, Edward VII...
How it got to be 'World War' is confusing..
Russia got involved to curb Prussian expansion in the East... (The Russian revolution caused the disintegration of the Russian army so that kind of petered out in 1917 - 18)
Then there were tensions over India, and rival Imperial influences in the middle East..
'British Empire' countries sent troops to help the British..
The USA came to the aid of the Brits...
When you look at it now it seems inconceivable that European countries should be trying to grab land from their neighbours, or control other countries in order to exploit their resources. (though now we go to war for different reasons)
A good history is what you really need, jura