I’ve seen this question enough times where I think it’s
important to differentiate between a few things.

Misconception #1: That nations are a reflection of their governments.

This couldn’t be the more opposite. Countless times
throughout the series, the nations have been shown to do things against their
will. Right from the get-go, this rule is established in chapter 1 of World Stars.

The nations may not agree with their bosses, but they
nonetheless accept their fate, resign, and obligately listen and do what they’re
told–not that it makes it right, nor am I justifying it.

Likewise, the nations represent their people. Yes, they’re
political in the sense that they’re forced to work for their governments, but
it’s important to remember that they’re cultural and historical
personifications.  

They don’t represent the government. The nations represent
their people in whole; the government is only a small part of that.

In other words, likewise to the stereotypes they embody, the
nations are a symbol of their people’s cultural identity.

Think of it this way: when you identify yourself with a
national identity, is it predicated on the negative actions of your government?

No.

Then why apply the same faulty logic to the nations?

What the government does is not a reflection of that
nation’s culture. That said, it wouldn’t make sense to attribute the actions of
a political minority onto a cultural personification that represents the
majority. Not to mention that again, the nations are often forced to follow
their bosses’ orders.

Misconception #2:
Political Enemies = Personal Enemies

Remember that the nations themselves are people too.
Regardless of the wartime alliances they get involved in, they’re more often
than not able to transcend over them. While war may impact how they formally relate
to each other, it doesn’t impact the past that they’ve shared and experienced
together (spanning across decades, centuries, and sometimes even millennia).

So, while they may be enemies during war, when fighting
isn’t necessary, the nations will still interact and see each other as friends
above all else. Political relations between national leaders is more orientated
in the present, whereas due to long-established and developed relationships, the nations are more influenced by the past. However, that doesn’t mean that they hold grudges either. It just means that they have more to consider. 

Misconception #3: Tense Relations = Incompatibility

Like I said above, the nations are humans too. Yes, their
countries may not have had/have the best relations, but that doesn’t
automatically mean that the respective personifications won’t get along either.

The best example of this is how Russia and America interact
with each other, especially in modern
times. They may not be the best of friends, but they still share a considerably
familiar and comfortable relationship.