what is the main conflict in the book? is it internal or external?

The reader sees that there is a great deal of external conflict, especially between Lennie and various people such a Curley's wife, whom he ultimately kills accidentally. George, in his trying to take care of Lennie, often intervenes in the "man vs man" conflicts because Lennie's limited abilities causes him to miss the nuances of life among groups. George does have some internal conflict, since he has to decide whether or not to kill Lennie. He makes the decision to "put him out of his misery" which has not happened yet but would be "external" if he had to face the "mob" after Curley's wife's death.

Of Mice and Men has two main conflicts. The first conflict is internal (Man vs Himself). George's feelings of responsiblity for Lennie are a source of turmoil. He loves Lennie and has promised to care for him, but this promise often leads to trouble and puts George in jeopardy. George can't move on, and he can't leave Lennie behind.

The second conflict in the novel is Man vs Man; it is an external conflict. There are various characters facing this conflict at different times; we have Lennie and Curley, Lennie and Curley's wife, and most the most prominant of all..... George and Lennie's conflict with society. The first two of these external conflicts are physical and sometimes violent. The last is a case of true isolation and the impossibility of fitting in.

There are other external conflicts in the story if you look thoroughly through the text.

what was the internal and external plot of of mice and men