I'm finding a lot of DF seems to be about maximising what you can get done over a certain period of time through thinking beforehand about what your objectives are.
Whether it be building enough beds to keep your migrants in reasonable living quarters, setting up your workshops so they run smoothly, training your military up adequately for when you inevitably need them or preparing coffins for when said soldiers rapidly become mangled tin cans...it's all down to forethought and planning.
After losing a fortress to an enormous Undead siege, it struck me that I know very little about building good defence into my fort from the ground up. Much of my architecture, for example, didn't lend itself to fortifications. That made it tough to establish good areas to fight in when they became essential.
My latest fort, I've made a slightly more defence-oriented start by parking my entrance in a valley. I've walled off unneccessary pathways through the valley and then dug a trap-filled channel to my main opening drawbridge. Additionally, this time I dug my living quarters straight down until I hit a plentiful amount of magnetite, allowing far more prompt acquisition of advanced metal traps and gear. A lovely side effect was that the Giant Axe Blades I have as surplus seem to be loved by the Dwarf caravan, and I got a ton of goods for them.
The tradeoff was that I nearly starved/died of thirst completely, because my still production was very wonky. Tense stuff 
How do the veteran longbeards here like to approach these problems in the early game, when you're often trying to get your booze/food/beds/offices set up as a priority, yet need to lay a groundwork for later? I think it'd make for an interesting discussion 
I am a believer in military embarks, because unless you flood yourself or get bored, you are only going to have problems with invaders, and while these problems are solvable a number of ways, running your guys out and killing them is very satisfying.
I have been playing the same embark over and over for months. It's got steel, tin, tetrahedrite, high magma, and a necromancer tower. Sometimes I get an undead siege before the first caravan. I get ambushers in year 252, sometimes a small goblin siege in late 252, and sometimes a huge goblin siege in late 253.
I start two military on day one, and two more when the first migration wave happens. Anyone who arrives with military skills gets drafted unless they have a labor that I need. I train dwarfs in pairs, without shields, which is the fastest way I've found to skill them. My original 7 have +5 shield/+5 armor. If I could change this, I would bring 6 in that config plus one that is mechanic/weaponsmith, because I normally use iron serrated disks as trade goods.
I know where everything is, so my goal is to get metal going and gear a couple of military.
I build exactly one entrance to start. It's 3-wide, very short, and has an elbow and a z-level change. The elbow has three cage traps, and hooks to the barracks. There is no way into the fort that doesn't go through the barracks. I build the drawbridge and hook it up. I build a hospital and put soap in it. I get this all running before the first caravan.
I bring enough booze to get to the caravan, but this time I didn't bring enough food, so I get the turkeys going ASAP (6 hens + gobbler) and make a couple of meals out of eggs.
If I get an undead siege before the first caravan, I don't want to greet it with a naked military with a training axe, so it's my goal to have a pair of competent dwarfs in bronze or iron armor, with a metal axes, usually steel ones. 2-4 dwarfs in steel can destroy any undead siege as far as I can tell; you just have to fight them inside away from necromancers (they usually walk into the entryway cage traps since they move faster than their minions).
This is normally good enough to solve the undead siege, if it happens.
After the first caravan I channel out an area for above-ground plants and roof it over. I bring my animals inside if I haven't already, so the only thing above ground is trees.
When the Spring migration wave comes I have a couple of axe lords, and start to work on my economy, i.e. make booze and food and get farming and clothes going.
If I get an ambush during 252 it's going to walk into the barracks since nobody is outside after mid-year, and the two dwarfs there will kill it. They can also kill small sieges and any undead siege, and often I have four of them by the second dwarf caravan.
After the second dwarf caravan I set up a trap hall, which I can run larger sieges through. Larger sieges come with animals that tear arms off, and that's bad, so I just murder them in a 3-wide weapon trap hall. I have never had a caravan come during a siege, so I just close the normal entrance and use the trapped one the moment the siege starts. The trap hall opens into the butcher shop, for disposal of mounts.
This is enough to survive anything until all of the random scraps of military I've picked up amount to something, but I've never let the game go that far. I usually think of something to improve, and start over.
The problem I'm having recently is getting simultaneous caravan arrival + siege, and I've tried numerous ways to solve this, and none of them are both appealing to me and fix this.
But my fort won't die, ever, as far as I can tell, unless I unleash hell or do something stupid.
TL;DR: Military is everything, and you should starve and live in the sand rather than delay military training and equipment.