I can't find a way to get past this mission. The only part that I am able to get to is the North route, and, right after that, I somehow fail the mission. Can someone please help me?

I am stuck on mission 5 now, myself. IF you are still stuck on this, all you can do is change-up your strategy. I have not beat it yet, but I'm getting closer with each try. (tried 3 times now.)

*****HINT***** DO NOT READ BELOW UNLESS YOU WANT A HINT!**** To get past this little chapter, I would consider not splitting up your forces (at the start). It seems to have helped me, but I'll have to get back to you -if you still check this- on if it works towards actually completing it.

This is an old old old question but by coincidence I have just finished that mission so if anyone else is stuck at this level here is what worked for me

[Spoiler follows]
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Move all of your troops South first and fight a minor battle there. This triggers a warning that you have been detected and that the convoy has gone North. Then back track with all of your troops to the point where the North route and south route join. Stick your infantry in the APC and you should have plenty of time to get there before the slow moving convoy arrives. Kill the escorts and blow up the slow moving transports to complete the level.