Difference of using と and と一緒に as "taking action with someone"


‎In Lesson 4 of Genki 1, it states that one of the meanings of と is "together
with". e.g. メアリーさんはスーさんと韓国に行きます。

However, the adverb of "together" is 一緒に and when と is added it has a nuance
of "taking action together with someone". e.g. 私は彼と一緒に学校にいきます。

What is the difference between using just "と" and using "と一緒に"?

よろしくお願いします。




と is like "with", a particle to mark other parallel agent(s) involved in an
action. That means:

> メアリーさんはスーさんと韓国に行きます。  
>  メアリーさんはスーさんと一緒に韓国に行きます。

They basically mean the same thing, but:

> アメリカはソ連と戦います。 _USA fights with USSR._  
>  アメリカはソ連と一緒に戦います。 _USA fights together with USSR._

～と戦う is just like "fight with", where you don't know what comes after is a
friend or foe. Using と一緒に makes sure that it is in your party. (Note: Japanese
has no equivalent of "fight against", so ～と戦う tends to mean the opponent.)

The same applies to ～と連絡を取る "make contact with" or ～と向かい合う "face with".

