….and the slaughter of Indians during 1800 was not pointless and unjust ?
The slaughter of Indians in the 1800s does not justify American intervention during WWI. Allied victory in WWI did not make the world a better place. It did not result in self-determination, the end of militarism, or any of the other idealistic principles for which Woodrow Wilson said he’d fought. Its main short-term effect was to allow France to exact a petty, unjust, cruel revenge against Germany for its victory in the Franco-Prussian War. The longer term consequence was to rob Central and Eastern Europe of its ability to resist Soviet invasion. According to former Soviet intelligence operative Suvarov, the main reason for the Spviet Union’s invasion of Poland in 1919 was the desire to continue on into Germany. Germany was weak, disarmed, and (in 1919 - '20) largely communist anyway. The German people had been partially radicalized by the famine conditions the Allies had created.
Fortunately, that particular Soviet invasion failed, due to the courage of the Polish Army, and due to the fact the Soviet Union was still in a state of civil war. But a disarmed Germany could not forever escape Soviet occupation. Germany’s only long-term hope was to elect a right wing leader–a militarist–who would match the Soviet military buildup with a strong military of his own. Unfortunately, the political circumstances the Allies created in the afermath of their victory were such that only a radical leader would throw off the unjust Versailles Treaty and seek to return Germany to a position of strength. It’s deeply unfortunate that the particular militarist Germany chose was a rabid anti-Semite and anti-Slav. Had WWI resulted in a tie or an Axis victory, Germany would presumably been able to maintain a strong military presence in the post WWI era without politically radicalizing itself. That combination (a strong German military and no radical anti-Semitism) would have yielded better long-term results than anything one could expect from the Allied victory.