Learn English – What’s the difference between “grammatical mistake” and “grammatical error”

Reasonable means that whatever decision was made is appropriate given the particular circumstances, and most would regard it as so.

Justified means that whatever decision was made is done for a good reason, in the interest of fairness.

There are times the words could be used interchangeably, and contexts where the difference is subtle. It all depends on the nature of the request or issue at hand.

I might use "justified" if I was angry about something, and was acting in response:

Was it polite to skip their wedding after they insulted me like that? No, but I think my decision was justified.

I might use "reasonable" if I think everyone would agree that there was nothing else that could be done:

Was I happy about missing their wedding so that I could go to my mother's funeral? No, but I think my decision was reasonable.

But there are contexts where either word will work just fine:

You said that you'd deliver the cake before noon, but it didn't get here until 3 o'clock. I think my request for a refund is reasonable/justified.

In that sentence, reasonable would work because most people would agree that a refund would be appropriate given the circumstances, and justified would work because the request is fair.

The two terms are closely related, and even interchangeable in many cases. However, the two words focus on different things. Alliance is more about mutual interests or benefit, while coalition is more about doing some action.

The use of both terms is not limited to the political context. They can be used in other contexts: military, financial, commercial, technological, and so on. Having said that, I agree that coalition is used most often in the political context for "a temporary alliance of political parties forming a government or of states" (see below).

Here are their definitions, according to Oxford dictionary:

alliance (noun)
1. A union or association formed for mutual benefit, especially between countries or organizations.
1.1 A relationship based on similarity of interests, nature, or qualities.
1.2 A state of being joined or associated.

coalition (noun) A temporary alliance for combined action, especially of political parties forming a government.

Here is a quote from Get Them On Your Side by Samuel B. Bacharach (chapter 5):

Formally defined, a coalition is a politically mobilized collection of interest groups or individuals committed to achieving a common outcome (i.e., resistance or change). Through political mobilization you create a group that has some sense of shared goals and/or a sense of connected interests. A coalition is an alliance for joint action.

In short, a coalition is an alliance for joint action.