What was the main reason that britain entered the war

because it was an ally of france

because germany invaded neutral belgium

because it supported slavic nationalism

because archduke fracas ferdinand was assassinated

Germany invaded neutral Belgium--Great Britain viewed an attack on a neutral country an act of aggression.

Germany invaded Belgium as a quick means to invade France. Belgium should have been avoided being a neutral country but Germany took the risk so they could attack France quickly and then turn to fighting Russia. Great Britain being in an alliance with France and Russia, fought on their side in the war.