Slitherine

I am playing as Axis and attacking in Middle East. When my units cross the red sea they lose supply. I have a clear path back to Tobruk but i have not captured Port Said yet. Why do I lose supply?

...You actually answered your own question. You MUST capture Port Said to gain control of the Suez Canal and allow supply across it.

Thanks. I have Port Said now but I still get periodic out of supply turns for my units as they push toward Masul. I bypassed Cairo and left the UK Garrsion there but they aren't cutting my supply line. What else causes this situation?

The Desert is always out of supply to stop you skipping round it unhistorically.

Medium supply sources, which is all that are available in North Africa and the Middle East, have a supply range of 20 hexes. So if your units move beyond this range, they are immediately out of supply, unless on a coastal hex with a surface naval unit adjacent, or by a surface naval unit with an unbroken line of controlled ground/coastal hexes from the naval unit to the ground/air unit drawing supply.

The only unlimited supply sources are major capitals, which have a supply range of 55 hexes. Keep in mind that supply cannot be traced across water or uncontrolled hexes.

See page 33 in the manual for the full rule set.

Desert hexes have low supply (black circle) and not no supply (red circle). This is the same supply level as from a BB, CV or DD. From low supply level you can repair 1 step per turn.

The desert in the Middle East makes it harder to advance quickly. If you start in a desert you have low supply and your movement range is much lower. You can only repair 1 step per turn so it's a good idea to place your units in clear hexes if possible.

ok, I got it now. Thanks for all the responses.