Front Mission 1st counts with two different campaings to play through, the OCU side is the original story of the Snes game, and follows Lloyd/Roid and his mercenary company the Canyon Crows during the event that put an end to the 2nd Huffman conflict. On the other hand the UCS/USN side puts us on the shoes of Kevin a UCS official that gets pushed around the 2nd Huffman conflict by his command and the mysterious Nirvana Institute.

The game recommends playing the OCU side first, and lets the UCS side for more experience player due to maps being somewhat more difficult. But an issue with this approach is that the OCU side story while interesting at a plot level, really shows its age and the dialog and character interactions aren't as good as the UCS side that was developt later.

So the purpose of this guide is to make a simple and spoiler free guide to play both sides at the same time (in different save files, please, don't forget to use different save files) in the chronological order of events in a way to spice things up.

When selecting NEW GAME

Play the tutorial (If you want)

Pick the UCS side. Play:

U-Mission 01 - Killing Drones
U-Mission 02 - Destroy the Base
U-Mission 03 - Border Patrol
U-Mission 04 - Crazy Wanzers

Notice for FM purists:
After Completing U-Mission 04 and saving, ideally one would play the O-Mission 1 and get back to the UCS side to get context on the factory scene, but... if you play O-Mission 1 you can only save after a scene that says "1 year later", so for this guide we will continue with Kevin during this "missing" year, and consider O-Mission 1 as a flashback of sorts.

U-EX-Mission 01 - Destroy the chopper
U-Mission 05 - Destroy the Radar
U-Mission 06 - Destroy the SAM Launchers
U-Mission 07 - Save Supply Trucks
U-Mission 08 - Take the bridge
U-Mission 09 - Take the City Hall
U-EX-Mission 02 - Sparring Match
U-EX-Mission 03 - Save Supply Trucks Again
U-Mission 10 - Desert Recon

After completing U-Mission 10, watch the scenes and save after the scrolling text, at this point we got back to the menu.

Start a NEW GAME and pick the OCU side.

Play:

O-Mission 01 - Factory Recon
O-Mission 02 - Save greedy Mercs
O-Mission 03 - Recover Plane Cargo
O-Mission 04 - Scort the convoy

After completing the mission, save then load the UCS side:

U-Mission 11 - Hospital Recon

Back to the OCU side:
O-Mission 05 - Hospital Recon
This two mission happen almost at the same time, but I like to play it out like this, so you get Kevin's questioning what is happening and LLoyd side giving the explanations.

O-Mission 06 - Destroy SAM Launchers
O-EX-Mission 01 - Stop recon unit

Back to the UCS Side:
U-Mission 12 - Highway Recon
U-Mission 13 - Railway Attack

See all dialog scenes, save before heading out to the next mission.
Load the OCU side:
O-Mission 07 - One of the most memorable OCU missions, no description needed.

Back to the UCS Side:
U-Mission 14 - Defend the bridge

Back to OCU Side:
O-Mission 08 - Clean up the city.
O-EX-Mission 02 - Defend the dam
O-Mission 09 - Ghost town recon
O-Mission 10 - Save greedy mercs again

Back to UCS Side:
U-EX-Mission 04 - Protect the building
U-Mission 15 - Rescue stragglers
U-Mission 16 - Secure Landing Zone

Back to OCU Side:
O-Mission 11 - Crossing the river

Back to UCS Side:
U-Mission 17 - Protect the prototype
U-Mission 18 - Survive the ambush - Complete the mission and save when possible.

Here we have another hard switch to make, because U-Mission 18 clearly happens before all the next OCU missions, even if it puts a tight timeline of 2 days for mission 12 to 15 to happen.
The issue comes that again, the scene after battle ends doesn't allow to save at any point before the knowing that the war ends during a one week time skip, and since Kevin appears on Fort Monus, it means that dialog happens before O-Mission 16.
In any case, I prefer the dialog to play like this, so you get to this point on Kevin's arc, and then switch to see how the war actually ended while he wasn't watching.

Back to OCU Side:
O-Mission 12 - Destroy the Train
O-Mission 13 - Desert Recon
O-Mission 14 - Save the deserter
O-Mission 15 - Destroy the Fortress
O-Mission 16 - Take the city
O-EX-Mission 03 - Clean up the city
O-Mission 17 - Scort the chopper
O- Mission 18 - Defeat the enemy
O- Mission 19 - Defend the camp
O- Mission 20 - Weird Allies
O- Mission 21 - Mountain Fight

Back to the UCS Side:
U-Mission 19 - Scape

Back to OCU Side:
O-Mission 22 - Fight at the old mine
O-Ex-Mission 04 - Hidden Lake
O-Mission 23 - Factory fight

Back to the UCS Side:
U-Mission 20 - Outside the institute

Back to OCU Side:
O-Mission 24 - Factory Recon Again

Back to UCS Side:
U-Mission 21 - Outside Port town
U-Mission 22 - Stop the plane
U-Mission 23 - Final Mission

Back to OCU Side:
O-Mission 25 - Old Friends
O-Mission 26 - Capture the warehouse
O-Mission 27 - Defend the warehouse
O-Mission 28 - Battle at the port
O-Mission 29 - Get to the chopper
O-Mission 30 - Final Mission