My experience playing Serbia in the 1914 scenario...

[Shinkuro Yukinari]

Attempt #1: Focused on upgrading my units and holding both myself and Montenegro while only giving up on Šabac.
Given how Montenegrin troops are attrocious and they don't get the mobilization event like I do, they die within 3 months, not to mention Germany reinforcing their lines ASAP and ultimately attacking Belgrade with 20 divisions on my 6.

Attempt #2: Montenegro is hopeless, let's try to hold Niš, Bor and Belgrade. Seems to be going well, Austria isn't as hellbent on murdering me and focuses more on Ru...
41 divisions encircled in Warsaw.
No matter, imma get supplied through Romania and hold on until Greece joins and then we can pinch a bunch of Germans thanks to Albanian neutrality.
2 years later...
Russia has fallen in early 1916, I still manage to hold, although I am running out on manpower.
Romania has joined the war and manages to push pretty well, we are close to taking out Bulgaria!
*Romania signs Armistice*
HOW THE HECK?! NO ONE WAS EVEN NEAR BUCHAREST!
Ok Shinkuro, calm down, they won't annex her completely, you can still use Romania for supplies...
6 months later I die

Attempt #3: Albania is horrendous but much more defensible, let's be more willing to retreat there...oh wait they said No.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

 

[FunnyDude]

git gud noob.

 

[Dr. Intolerance]

Attempt #1: Focused on upgrading my units and holding both myself and Montenegro while only giving up on Šabac.
Given how Montenegrin troops are attrocious and they don't get the mobilization event like I do, they die within 3 months, not to mention Germany reinforcing their lines ASAP and ultimately attacking Belgrade with 20 divisions on my 6.

Attempt #2: Montenegro is hopeless, let's try to hold Niš, Bor and Belgrade. Seems to be going well, Austria isn't as hellbent on murdering me and focuses more on Ru...
41 divisions encircled in Warsaw.
No matter, imma get supplied through Romania and hold on until Greece joins and then we can pinch a bunch of Germans thanks to Albanian neutrality.
2 years later...
Russia has fallen in early 1916, I still manage to hold, although I am running out on manpower.
Romania has joined the war and manages to push pretty well, we are close to taking out Bulgaria!
*Romania signs Armistice*
HOW THE HECK?! NO ONE WAS EVEN NEAR BUCHAREST!
Ok Shinkuro, calm down, they won't annex her completely, you can still use Romania for supplies...
6 months later I die

Attempt #3: Albania is horrendous but much more defensible, let's be more willing to retreat there...oh wait they said No.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

With the new update to DH, reinforcement also slowly upgrades so I'd set all your infantry to upgrade to mountain fighters, as there are lots of mountains around.

Militia are quick to build and have greater attack bonuses crossing rivers so you could try that from time to time, as Serbia fielding the first ever corps of frogmen. Give them engineer brigades.

 

[Firestorm]

Serbia is hard. If CP throws full weight on you instead of on Russia/France then you're pretty-much screwed. Otherwise there might be hope.

Agree with Dr Intolerance. Militia is good, offensive strategy is good. Serbia is one of few nations where mountain infantry is good.

 

[Shinkuro Yukinari]

I suppose giving up on Šabac and even Užice is ok since I can defend on a river, but can I hold the rest of the country with mountaineers?

 

[Shinkuro Yukinari]

Dumb question
How do I set my units to upgrade to mountaineers? I checked in the UI and I only found options to disable reinforcements and upgrades
Also to note is that in DH Full Serbia starts with Light Infantry 1897 and needs 2 researches to get Mountaineers

 

[Lucifer]

Dumb question
How do I set my units to upgrade to mountaineers? I checked in the UI and I only found options to disable reinforcements and upgrades
Also to note is that in DH Full Serbia starts with Light Infantry 1897 and needs 2 researches to get Mountaineers

First, you need to have the technology. Once it's done, select the unit, then under its location you will see a menu you can expand (with a reversed black triangle on the right). From there, you will be able to select a new path for the unit.
This is a very useful feature. For example, as Germany it may be difficult to built PAR in time before invading France in 1940 unless you are willing to rush a tech with the negative consequences in term of amount of tech researched. With this feature, once PAR are researched (without rushing the tech), just upgrade a regular INF to PAR, it is much faster.

 

[Shinkuro Yukinari]

First, you need to have the technology. Once it's done, select the unit, then under its location you will see a menu you can expand (with a reversed black triangle on the right). From there, you will be able to select a new path for the unit.
This is a very useful feature. For example, as Germany it may be difficult to built PAR in time before invading France in 1940 unless you are willing to rush a tech with the negative consequences in term of amount of tech researched. With this feature, once PAR are researched (without rushing the tech), just upgrade a regular INF to PAR, it is much faster.

Thanks. Now let's pray to whatever divine entity there is out there that I don't get Steamrolled out of Belgrade within 2 days

 

[Soul of Darknes]

Win this and then you can try Luxembourg