I have often wondered why my trade routes were destroyed or interrupted. Now I have seen when my ally suddenly destroyed a lux resource on a territory of an Independent People. As a result, my Trade Routes that passed through that territory were destroyed. Wow I thought to myself, just a small lux and such a big effect. I analyzed it further and here are my observations in the table:  

From this you can learn that it is important to know where to place your own Trade Routes. You often have a choice between sea route and land route. One should rather avoid the IP territories, as it brings great risk with it. 

Every aggressive action with IP is considered as a small war, which lasts only for a very short time. If our import route is destroyed by another player we get grievances.  

For newbies I did a lot of tutorials but unfortunately only in polish (Youtube Paxis)

Otherwise look at the forum here or ask at the Ampitude discord.