associate, ally, combine, unite这一组词都有“联合”的意思。

associate v.交往，结交；联系，联想。多指平等友好的合作，后接with，表示与某人交往或合作，后接in表示在某个方面合作。

We associate with our neighbors at church.
在教堂里，我们同邻居交往。

ally v.使结盟，使联姻，多与with搭配。

Several political parties allied with each other against higher taxes. 
几个政治党派联合起来反对提高税收。

Another ally can do this but not that.
另一个盟国能这样、又不能那样做。

He has also proved, far beyond the call of duty, his affection for the US and his loyalty as an ally.
他同时也证明了，他对美国的热爱和作为一个盟友的忠贞，都远远超过了职责要求。

I hesitate to even mention this because I believe in the power of one, but you can find an ally to help.
我很犹豫是否提到这个，因为我相信个体的力量。 但是你可以找一个同盟者来帮忙。

combine v.联合，化合，混合，多指为了某个目的将不同的人或物混合而成一个整体。

The girl combined sugar, butter, and flour to made the dough.
小姑娘把糖、黄油和面粉混合到一起做成面团。

unite v.联合，合并，团结，尤指不同的事物或人组成统一体，强调结果的一致性。

The nation united against its enemy. 
国民团结一致共同对敌。