As Eric Hobsbawm explained in <em>The Age of Extremes</em>, World War I happened when the European Empires were crumbling and falling into pieces. Liberal theorists opposed to war as a means of Empires to gain power over others. The economic costs clearly disfavored war but there were also the ethical and moral aspects that gained momentum prior to WW I. These aspects materialised again in the 70s (Vietnam war) when more people started to protest against the human costs of war.
the Monro doctrine was a statement saying if you attack any of our neighbors and wish them harm we will defend them this was a statement used to say hey america over here we putting our foot down and saying no to colonialism this also kicked of the war of 1812