Re: if the US staye out of WW1
The Central Powers were already losing when the US joined. In the war, the US were hardly needed. Even if the USA stayed neutral, we would have won.
I can only concur. Although US sources make a lot of the US entry into the war in April 1917, the truth is that US troops did not engage the enemy in any numbers until the Battle of Catigny in May 1918, previously having been consigned to support and rear roles due to a slow training regime and lack of equipment, except for the odd pride-saving attachment to French and British units.
May 1918 was after the German's spring offensives had failed. While the Germans achieved the greatest advance since 1914, it was these Spring Offensives that broke the German Army. Apart from the best of the specially trained Stormtruppen being ground to mincemeat, those that did overun British and Commonwealth trenches found an abundance of food and equipment that they could hardly dream of. Defeatism spread through the German Army, not least amongst the senior officer caste. The allied stop of the offensive and the counter-offensives of May to August 1914--especially the Canadian-ANZAC spearhead at Amiens did the rest. Hindenberg and Ludendorf essentially cracked.
The US involvement was esentially psychological. The Allied press made much of the 2 million troops that had arrived in France and contemporary photos show young, tall, strong, healthy looking men compared to the worn-out dregs left to the British and French (and Germans).
A lot of criticism has been made of the refusal of the American command to heed British and French advice on tactics and sacrifice their troops as the Allies had done from 1915-1917; but maybe that also influenced the Germans who in August 1918 just wanted it all to end.