Having a state of war with Italy is pretty much your best bet.
It's best if you watch Marco's videos on the subject since, as far as I know, it's his idea, but it's quite effective.
Basically all you need to do is hold the line (let Belgium die), Germany will have only 1-2 divisions facing you once Belgium collapses, then you push into former Belgium to try and get yourself some nice river lines.
The reason you let Belgium die is because there's a "bug" in the game where if your units sit in allied territory they consume resources as if they were moving (so let's say that's roughly at x2 the normal rate). That means if you have any armor as France (which is basically the only way not to die) you will end up running out of gas within 1-2 years, and then your units will become useless - if you let your units sit in Belgium before its collapse. Furthermore, if you were able to defeat Germany and push *through* Belgium, being as they're your ally, all the territory you took from Germany would go to Belgium and not France.
He then takes the navy, and some spare troops to Japan and conquers the home Islands for a massive IC/MP/LS boost, that's not necessary unless you're playing at a very high difficulty or like the game to be easy lol.
The crux is, once Italy declares war on you (or you on them when you're ready) you knock them out in the Med by taking Sardinia, Sicily, and Libya - Germany will pour troops into Italy to reinforce the Alpine segment of the Maginot. Importantly, with troops in Sicily, many troops will go to the bottom of the Italian boot to block the strait - it's possible for you to hold there with a minimal investment of force (and even if they do manage to cross it won't matter soon enough...). You can also take Norway back if you wish, but it's not essential.
You then land a sizable force in Albania, and use that as a base to attack Venice. Venice will be lightly garrisoned, so the landing should be easy. Then you secure the only remaining Italian port East of Venice, and shove your army into the alps and make to establish a line to the Swiss border.
Now all of Germany's troops in Italy are being supplied from Rome....
You then make an amphibious landing to the port just North of Rome, and assault Rome... Now all of the Italian and German units inside Italy are going to run out of supply.
After you mop them up, you make a landing in either Denmark or the Netherlands (or both if you've got the troops) and that should thin the Germans out enough for you to start pushing them back economically - off of your landings in the Netherlands, (possibly) Denmark, your Belgian line, and your Italian line. Note also that Italy should be a GiE by now lol. You should be able to handle the Germans by then - but the possibility of defeat is still real if you end up taking a lot of hard fights and run yourself out of manpower. Divisions may end up shattering.
The other essential point to this strategy is to build as many HARM brigades as you can and attach them directly to your infantry brigades (what the French did IRL - which is often cited as a major contributing factor for their defeat). Ideally your main divisions will be 1xHarm + 3-4xINF brigades. As long as your HARM armor is up to date, the Germans should have a very hard time dislodging your divisions because of the wacky way piercing is done in this game. After you thin them because of the loss of Italy and the units in Italy, you are able to push them without running your MP pool dry (or at least not losing the war due to losing too many divisions to lack of MP).
The reason HARM works so well for France, and why their piercing makes your INF nearly invincible to boot, is because France starts the game with the very best armor techs in the game (they start with Medium armor already teched up). So it's *very* possible for you to start constructing HARM at a very early date if you research your armor ahead of time, and use the building of lesser tanks to boost your practical to make the research less expensive. In this way you can virtually guarantee that your HARM is 1 tech cycle ahead of whatever Germany's gun techs are - so they will be completely unable to pierce your HARM lol. Which makes your divisions *very* difficult to defeat. But don't get cocky, they can still throw units into you and bomb you enough to cause major damage, and to have you lose territories if you're not careful.
If you want to make it somewhat easier you can also spend some time influencing or couping Hungary to prevent them from joining the Axis early - less divisions to have to deal with.
I should also point out if France is too belligerent in this period bad things may happen (say you do the above quickly after the war breaks out and you defeat Germany before 1941, and go about attacking all of the Axis minors who didn't declare war on you - the USA will not join the allies and will instead join the Comintern because France will pose 200+ threat to them lol).
