Parshat Ki Tisa
Hashem is telling the people that He is bringing them into the Land of Israel and they must chase out the nations...In 34:15 Hashem warns them, "פן תכרת ברית ליושב הארץ ... וקרא לך ואכלת מזבחו" - lest you make a treaty with the inhabitants of the land and turn after their gods and they will slaughter to their gods and will invite you and you will eat with them.
It starts off with something innocuous like getting ogether for coffee or dinner with the nice neighbor. What's wrong with eating with him - the food is kosher! he shechted the animal according to our specifications specifically so he could invite me over!! There's nothing wrong with it.
I hear myself making the above excuses for all sorts of things that are technically not wrong.
The problem is that one thing leads to another. You start off by eating dinner with him and the Torah says that it leads to worse. You develop a relationship, you get together often, your kids become good friends and end up intermarrying, and eventually your descendants are worshipping other gods. All because you made an excuse to have dinner with the guy.
That is why the Torah warns us to not to make a treaty with them and eat with them. One thing leads to another.