From: Republic of Cascadia
@ Chernobyl: Yes Serbia is defendable but you have to be very flexible and adapt to CP moves / overall situation.
ThisEndUp and Tanaka are right about CP early involvment down there: it means weaknesses Entente need to exploit on other fronts.
That said, if Germany use a "mobile" army (HQ 4 corps 1 artillery) strategy to deal with trouble on foreign fronts (OE, Italy, Serbia, Greece...), you can just buy time with Serbia.
Standard strategy is to entrench on turn 1, reinforce units on turn 2 and then just defend while slowly falling back using terrain to delay CP as much as you can.
If you're submerged on turn 1 like in your pic above, don't hesitate to sacrifice ground and retreat/operate to a shorter/better defensive line.
Try to entrench after moving or use ZoCs to avoid attacks/enemy units coming adjacent the following turn so you can entrench.
Sacrifice units you can't move/save far enough to gain some time / 1 turn.
You also need to have clear priorities:
1) Serbia severely lacks MPPs, meaning you have to manage your units very efficiently
- defend the center of the line with your corps, entrenched and attached to HQ, use detachments at the end of the line in difficult terrain and where you can afford to lose ground/an hex
- use extra units (detachments, weakened ones, unattached/foreign units...) to entrench in the back as a second line so you can retreat on prepared lines or switch units if you have a chance to attack
- never attack unless you have very good odds against exhausted AH units, even then it's better to just use your turn to reinforce your corps back to full strength
- doing that also limit the risk of messing up attachments
- if you have to lose units, try to lose detachments and lose them in full supply because you can afford to buy them back easily and they'll come back quickly
- as an aside, try to keep 50-100 MPPs available each turn to deal with unexpected losses/situations
- you have to hold long enough to benefit from all free units coming by event (volunteers and such)
2) Since you lack MPPs, limit research and try to max out the russian convoy as long as it lasts, disable the naval convoy if you can't protect it from AH raiders
- as research you need trench before anything else, reinforcing your units is a priority but put at least one chit there as soon as you have some leeway, it will work for you each turn
- only defending will make it easier to save MPPs for more trench chits and just level 2 will make things difficult for CPs without deentrenching artillery
- the typhus outbreak that decimates your units will also be more manageable
- it will also help to maintain the starting experience of serbian corps as long as possible, increasing their combat prowess (every little bit helps)
- that will also give you a chance to build some XP for your serbian HQ (once again, every little bit helps)
- infantry tech should be next if you survive long enough
- buy back starting research chit to gain some MPPs right away (to operate/save units or reinforce them next turn)
3) Belgrade is just a bump speed, Nish is far more important and more defensible
- don't hesitate to evacuate Belgrade on turn 1 (rail or move), same for Valjevo
- saving the two detachments on turn 1-2 is a big help as once reinforced/entrenched they can greatly stabilize your defensive line
- even if you have to lose them on turn 2, put them on supply 5 or more hexes so you can buy them back cheap right away
- Serbia has good defensive terrain and the more you retreat, the more the front shrinks so you need less and less units (5-6 at start then 4 then 3 then 2).
- fall back to a shorter front whenever you lack/lose units (especially corps)
- the main point isn't to save Serbia but to delay/deny CP use of rail towards OE as long as possible (so you can lose Belgrade, no biggy)
- there isn't any NM and only limited MPPs to gain for CP in Serbia so (Belgrade 8, Nish 8, mine 15 so around one mine at full strength once they reach max production, nothing much)
- it's more if you count every ressource in the area including Cetinje and Tirana but still only MPPs and nothing more than another mine
4) What about Montenegro and Albania ?
- I consider them as units providers mostly for extra detachments
- the hardest decision is wether or not to give the russian HQ to Montenegro, I usually say yes since it allows their corps to be combat worthy and join the main defensive line
- you can go for two serbian HQs but you'll hardly have the use of it since it's hard to maintain more than 6-7 serbian units combat ready at any time
- CP can kill Montenegro quite quickly if they really want to (though quite costly) even if you decide to use the montenegrin corps to defend the capital
- defense there is usually entrench detachment in capital from turn 1, other detachment next to it on the road or in Pec to block movement in the mountains (reinforce to 10 as soon as possible)
- the main problem with Montenegro is surrender rules are pretty tricky and their units can vanish at the worst possible time
- so never use montenegrin units to defend important hexes/places in Serbia if CP is going hard for Cetinje/Montenegro
- last thing: be careful if you leave Tirana empty, CP amphibious move is unlikely but still possible...
5) Keep an eye on Bulgaria / CP diplo investment
- you can ignore them and your back early on
- start planning for retreat / extra units when they are around 70% mobilized
- you only need two detachments entrenched in mountains roads on the border to delay Bulgarians, just watch out cavalry / forced march moves towards towns in your back
- if you're lucky and Bulgaria joins CP on your turn, advance those 2 detachments one hex on the road towards Sofia and entrench them there, it will give you some more cushion
6) Special moves: CP can bring reinforcements easily to Serbia but you also have that option if you are willing to pay the price
- send Entente units by sea to Albania/Montenegro (belgian, french or english units, italian ones later if they join you)
- bring Romania on your side early or... just invade it with Russia !
- massive diplomacy to block Bulgaria entry but that's a risky gamble
- push CP so hard on other fronts (mostly Galicia/Carpathia) that AH will have to stay quiet in Serbia
- amphibious landings on adriatic coast (from Italia or Albania if your fleet can control the sea)
All in all, it's a win whenever Serbia blocks the OE railroad well into 1915, just somehow survives or forces CP (especially Germany) to mobilize important forces to clean the area.
Below, possible successive defensive lines:
Dazo, this is a very impressive guide for Serbia! It belongs in it's own thread, or better yet..an amendment to the official strategy guide. Thank you.