It was a chain reaction. Long story short, the countries were pulled into the war because of agreements they had with their allies. This treaty is what caused the countries to enter WWI, to protect and defend their allied countries who were already in the war.
The supreme court tends to check congress more than the president because congress passes laws, which change the way the courts work in this country, so the supreme court is a major stake holder in what gets passed through acts of congress. Most of the time, a president is checked by the court through a bill they've thrown serious political muscle behind and gotten passed through congress. Great examples of the supreme court striking down presidentially endorsed acts of congress is the court striking down the Agricultural Adjustment Administration and the national Recovery Administration that FDR pushed for as part of the New Deal. This also nearly happened in recent times with Obamacare, where several components of the bill narrowly avoided being struck down by the supreme court. The supreme court can also check executive orders. The supreme court also struck down some elements of President Trump's muslim ban in the last month.
