think if it like an earthquake. you have tectonic plates drifting. when they run into each other, pressure builds up, until it slips, and you have an earthquake. geopolitics is similar. as countries develop economically, and their populations grow, their strength relative to each other changes. when the disparity between strength and geopolitical situation grows too large, you have an earthquake. for ww1 specifically, france was eager for a chance to retake alsace-lorraine from germany; germany was eager to take part of eastern europe from russia before russia industrialized and became too strong; austria wanted to destroy serbia to prevent ethnic separatism in their empire; and russia wanted to have greater strength in the balkans and eventually control the turkish straights.  the assassination of ferdinand was a pretext for austria to attack serbia. defending serbia was vital for russia to have power in the balkans. germany felt obliged to help their ally austria and also saw this as a crucial do-or-die chance to weaken russia. france saw this war as an ideal time to attack germany while they were distracted. uk joined in because they feared a hegemonic germany and also because being friendly with france was better for their overseas empire.
United States