After the Austro-Prussian war, and the Fraco-Prussian war. France, Russia and the UK saw in the raising of an united Germany as a potential menace. Thus after Russia entered to defend Serbia against Austria-Hungary and Germany; France honoured its secret alliance to Russia. In order to Germany get into France they invaded Belgium who bid for neutrality, so Britain defended Belgium's independence, then getting involved into the mess. I'm no expert at all. But I would say that after suffering such war together against a stronger enemy, they ended reckoning in each other as a strong ally.