Wig, the US sat out the first couple of years of each war for good reason.
Start with WW1. Britain entered the was in August 1914 and brought its empire countries along with it. But it wasn't until January 1916 that Britain started conscription; Canada not until August 1917, while South Africa, Australia & India never conscripted. So if you aren't compelling your men to fight, it didn't seem that serious to people in the USA--you haven't used all of your available resources so why should the US do any of your fighting for you?
Britain entered the war because if feared a German victory over France would result German domination of Western Europe--you were protecting your long-term interests, the members of you empire/commonwealth followed you to protect their interest in your mercantile system, a system that the US was not a partner in.)..so the US saw no threats to any of its interests until sinking of US shipping & the Zimmerman telegram. The US entered in April 1917, and began conscription in May 1917 (three months before Canada.)
At that time the US had a large element of German & Irish ancestry in the lower classes--the classes from which the fighting men would be drawn---Britain was not popular with either group. Conscription had to used in the US (Also in Canada, because the French Canadian generally refused to enlist--they didn't like the British or Canadian English either).
As for WW2, how you British & French mishandled the peace treaty pretty much guaranteed WW2 would come along. Most Americans saw getting involved in Europe again would be stupid after how the peacemaking was bungled. So before the US entered, Roosevelt made Churchill agree to the Atlantic Charter, a list of conditions that would be included in peace terms after the war ended. And even then, the US deferred entry, knowing that the British word is not often honored--as was the case when the British lied to the Arabs during WW1 about Lebanon. Indeed, Churchill started trying to recast the agreement shortly thereafter under political pressure. So still, it took something really dramatic, Pearl Harbor, to change the public opinion to support going to war--indeed, most men enlisting in December 1941 wanted to fight the Japanese, with no thought of fighting in Europe
You'd be wise to complain about Stalin-led-Russia being allied with Germany until mid-1941, all the while know Russia & Germany would eventually fight, but first Stalin wanted Germany to exhaust itself fighting France & England.