I chose the five sentences I was most doubtful about. I really hope you can check them.
1) The First World War, or the Great War as it used to be called, is rightly thought of as bringing cataclysmic changes in life, thought and social forms not only in Britain but in the whole of Europe.
2) Britain, like most other belligerants, had expected the war to be over in six months. On the contrary, the war became a long trench war.
3) The German fleet which, in 1914, had suffered serious losses, tried to force the British blockade off Jutland. The losses were the same, but the English fleet remained in control of the sea.
4) From July to November 1917,the British attacked the Germans in Flanders but lost 400,000 men against the 250,000 men lost by the Germans. 5. The British occupied Gaza and Jerusalem in December 1917. The war ended with 10,000,000 lives lost and 20,000,000 wounded men. The effects of the First World War concerned party politics, the labour movement, the role of women and social policy, too.
1. comma needed
2. It'd be more accurate to write, "... became a long war fought in the trenches."
3. OK
4. space after comma
5. comma needed
Overall, these are very good.