In Diplomacy, is it good strategy to have a primary alliance with a country on the opposite of the board?
Well, having allies is never a bad strategy, whether it is in Diplomacy or in real life wars. But the one thing I've learnt about Diplomacy is that an alliance never lasts long. There is only one winner at the end of the game. If you agree with the other country that Russia will be destroyed and that you will share the remains, it is a good idea. The question you should ask yourself is: "should I betray the other before he betrays me?"

Let me answer your question in a more generalistic way. Whenever two countries are allies in Diplomacy, each part should have these two strategies:


What will I do if we are allied? What do I want from my friend? What do I do when I have what I want?
What does my ally want? What do I do if I am betrayed? Is there a way for me to forsee that betrayal and use it to my advantage?


So basically, yes, it is a good idea to team up with a distant country, but until when?