No, I’m not referring to anything current. I’m referring to Germany in January 1945 or the Confederate States of America in January 1865. Your last realistic hope of reversing or halting the enemy advance has failed. You still have something like an army but It’s only a matter of time before you’re completely crushed. The enemy has absolutely no interest in negotiating: the only suggestion they have for you is quit now and stop taking casualties. Defeat means the absolute annihilation of your nation, your cause, your ideology, everything you consider good and right. If you’re the leader at the top, if you’re an officer in the army, if you’re a grunt in the field, whaddya do?
IF by leader you mean dictator or something like that , then your choice of poison , pistol or mercy of the court once you give the order to surrender.
If your an officer , then it means preserving whats left of your command and maintaining order until a formal surrender is established and the victors come calling.
The average grunt, basically stay alive and get ready for some sort of internment and demobilization.
And on top of that , it would really all depend on exactly whom is invading. IF Canada was invaded by the States and it was for all the marbles ,then I would have no big issues with a surrender. American forces and policy are a known commodity.
The russians, chinese or the draka , time to get in contact with the stay behinds and await orders.
Declan
Stop fighting, obviously.
EDIT: Well, Declan’s post is a bit more realistic than mine in its particulars, but I imagine that desertion for a private soldier, or surrender of your command from a general (even without approval from above), or immediate unconditional surrender from the head of state would be fine (possibly preferable) alternatives.
For the most part, unconditional surrender is what ends up happening. I don’t think a war has ever been fought that didn’t end in some kind of surrender, except when the goal was genocide (like under some of the Great Khans).
Cuss.
Many wars have ended without either side surrendering. (The War of 1812 or the Iran-Iraq War to name two.)
Many other wars have ended with a conditional surrender; one side makes concessions (such as territory) to end the war, but isn’t necessarily occupied, annexed, or massacred by the victorious side.
As Declan said, it depends on who you lose to. Everything from 'surrender peacefully", to ( if it’s the fortunately fictional Draka ) “strap cobalt jackets on the warheads and nuke the world till it glows.” Losing to the Draka is rather like losing to the armies of Hell.
Is taking up arms as guerrilla fighters and starting some king of insurgency off the table?
That never works.
I’m bracing for the whoosh here, but are you kidding? 
Personally, I’d be getting all Red-and-White Dawn on their asses.
If you want to see an interesting German-made movie about the last days of the Third Reich, check out Der Untergang (“Downfall”). It is told from the point of view of Hitler’s personal secretary as everything came crashing down around them. Very affecting and chilling in many places.
In all of human history, I can recall only one nation that surrendered under these circumstances. The Kaiser’s Germany. That did not work out well for them.
The Confederacy offers a more typical example. Jefferson Davis fled from Richmond and moved from place to place guarded by midshipmen from the Naval Academy, taking time only to issue a proclamation that they would fight on, freed from the need to defend the cities. Idiot!
So all in all, people are ornery cusses who will tend to fight on even when the cause is lost.
Not that I can fault you for thinking that way since an American invasion and subsequent annexation is not a viable future for either country.
Like it or not , if the prime minister or his designate gives a declaration of surrender, then your actions become ilegal and you become an unlawful combatant. If you concider the american armed forces to be the nearest thing to the draka , then this wont mean a dam to you , but it may be your neighbors that turn you in.
Declan
In the 1945 Germany situation, I’d have done pretty much what the Germans did, except that after a certain point (Feb 1945-ish) I woudn’t have tried to put up anything other than a VERY token resistance in the West, and thrown everything I had into fighting the Russians in the East, so that less of the country would have been under their control post-war.
I always thought that the Germans would have been better served not fighting the Huertgen Forest & Ruhr Pocket battles, and putting those troops in the East. They could have slowed the advance in the west some without putting up huge fights, and fought the Soviets much harder in the East with the half a million or so troops saved from capture and death at the hands of the Allies in the west.
Whaddya do?
Curl up in a corner and whimper
And on a more junior level, bump, if you are a German Officer in the West, you start surrendering. Much better to be a POW of the Western Allies than the Soviets.
In the East, you have the choice between the holding tactic of trying to buy time for the US/UK/Canadian troops to get as far East as possible, though it is unlikely they would go further than previously agreed. You can do a callous calculation and make sure that the troops facing the Soviets include all of those who simply cannot surrender - the units raised from Red Army prisoners, Ukrainians, other White Russian units, the SS divisions made up of collaborators from occupied Europe etc. Even if they surrendered in the West, they were screwed. It’s not a surprise that the last unit left fighting in Berlin was, I think, the SS Charlemagne division.
Was it not obvious to the Germans that the Allies on the Western front weren’t in any great hurry to get to Berlin, and instead had decided to let the Russians take the city (and suffer the casualties)?
Actually from what I’ve read, it wasn’t a sure thing at the time that occupation by the western allies would be better than occupation by the Soviets. Particularly, the Morgenthau Plan gave Germans reason to think that the Americans and British would be no more merciful than the Russians.
Given the number of Germans who had fought on the Eastern Front, and the treatment by the Germans of Russian PoWs and civilians alike, as well as the stories filtering back from the East regarding the behavior of some elements of the Soviet invading forces, I think for your average Wehrmacht grunt/NCO/junior officer capture by the Western Allies was seen as massively preferable at the time, regardless of the overall comparisons between occupation at a national level.