The Russians invaded Germany during WWII because the Germans killed twenty million Russians and destroyed one of every three buildings in some Russian cities and towns. You might recall from your history studies how the US also "turned around" and invaded Germany during WWII in spite of the fact that the US homeland was unscratched by the Nazi armies.just maybe you need to research the Soviet's history and its own civil war before WWII. And once the European Axis (Germany, Italy,et al) retreated in WWII the Soviet Union then turned around and invaded Germany.
Your history lessons also do not contain the information that we tried to stay out of both WWI and II until others attacked or threatened our own sovereignty.
If you have some proof of others threatening US sovereignty before the US entered WWI, please provide it. I am under the assumption we entered that war to protect Morgan's billions:
"The partners of J.P. Morgan & Co. and directors of First National and National City Bank controlled aggregate resources of $22.245 billion, which Louis Brandeis, later a U.S. Supreme Court Justice, compared to the value of all the property in the twenty-two states west of the Mississippi River."