My era of interest is 190-280 CE Ancient China, a civil war known as the three kingdoms following the collapse of the Later Han authority. Hopefully counts as ancient enough and I know something of the Later Han (25 CE to 220 CE though 190 CE the last Emperor became a puppet for a warlord) in the decades before that. You may be thinking of earlier times, ancient can cover a long time but maybe this will help

Starting with point 1:

A war can start for many reasons: A religious revolt, ambition, mistreatment, xenophobia, revenge, trying to get out from under the rule of another, fear summons back to capital will lead to your death, reactions to events at court, new overlord sleeping with a female relative and planning to kill you, trying to take advantage of events, "stop trying to undermine me", worried that the larger state will win out if you do nothing, resisting the destruction of culture by colonizing power, shifting power dynamics, trying to strengthen authority via military success and so on.

My era's plunge into civil war happened because of long-running tensions between the gentry and the eunuchs (who had become an important arm of various Emperors including against the gentry). During the reign of a young new Emperor called Bian, attempts to force out the eunuchs ended in fiery chaos as the desperate eunuchs assassinated General-in-Chief He Jin which resulted in certain officers and the Northern Army storming the palace to massacre the eunuchs. The imperial family fled, nearby general Dong Zhuo (summoned by He Jin to put pressure on the eunuchs) saw the flames and entered the capital then as the imperial family was returned, via trickery and military force managed to gain control of the capital. Figures at the capital like some of the Yuan clan fled to build support elsewhere, regional officers would raise forces and a coalition was built against Dong Zhuo. Once Dong Zhuo's power sufficiently receded to a regional figure, the regional authorities turned into warlords and turned on each other. Most nominally still "for the Han", forming alliances and battle until there were... three states claimed to be the legitimate Emperor with the mandate, all sides believing the land would be unified at some point. Until it was by the Jin dynasty following an internal coup of one of the three (sparking a period of internal conflict) and conquests of the other two, ending a near-century of disunity where there had been many wars.

A war doesn't just happen out of nowhere so unless the preparing for war has to happen in some extremely distant land, there is a chance that those about to be attacked will pick up something is coming. An invasion in response to something that has happened, long deteriorating relations, changes in an alliance, death of a key figure and other situations could give the defenders a sense that an attack might be coming their way and to be prepared.

Preparing to invade takes time. An army needs to be assembled and to march, plans made with general, perhaps diplomatic efforts required to gain allies abroad while at home persuading those less keen on the war, infrastructure like canal work to help keep the supply lines working. While unsurprisingly we know little of how intelligence was gathered, agents were used for information or to stir up trouble and maybe other means were found to gather information.

That isn't to say a surprise invasion is impossible, sometimes defenders can be blind-sided by a surprise invasion that they didn't foresee coming as they didn't know about the army build-up (for example if meant to be allied). Or even they did notice the troop build-up but felt the rival state would not attack due to their understanding of the state of the potential attackers. Even if they expect an invasion, an unexpected route (or timing) taken might catch the defenders having to scramble to meet the unexpected route of attack or fight before they were ready (or had been drawn elsewhere).

So what happened when an army arrived out of nowhere in my era? The defenders responded. To use an example, in the spring of 228 Shu-Han Prime Minister Zhuge Liang attacked a completely unprepared Wei dynasty, they had known about the army being prepared but doubted Shu-Han had anyone able to lead such an invasion and the man in charge of the north-west, relative Xiahou Mao, was not a man suited for military matters. Nearby commandries were unable to resist and switched to Shu-Han and there was alarm that the north-west of Wei might fall but within two months the invasion was defeated. By early March the Wei Emperor Cao Rui had decided to go to the city of Chang'an and would arrive in April while before then, he had sent armies under Cao Zhen and Zhang He who both defeated Shu-Han armies. Wei learned from this, the experienced commander Cao Zhen was left in charge of the area and defences were strengthened and repaired.

Where invasions in my period did come as "the defenders couldn't raise arms to defend themselves" would be at a local level and with the main army away. Most famously perhaps is the 219 invasion where Wu commander Lu Meng invaded to take Shu-Han parts of Jing in a surprise attack while Shu-Han commander Guan Yu was fighting in Wei parts of Jing. Guan Yu had thought Lu Meng was too unwell to lead an army, the tone of Wu figures was friendly and hadn't been aware of the army being prepared, instead he may have thought the Wu army was being restructured under a new commander. Lu Meng's army crossed the Yangtze disguised as merchant shipping and seized the watch-towers before they could be lit to act as a warning then attacked the land areas. Guan Yu did have defences at Gong'an and Jiangling but the commanders there had become uneasy due to recent rows with Guan Yu and surrendered quickly. By the time Guan Yu returned from his attacks on Wei, Jing was gone while Wu ensured any retreat to his lord in Yi was cut off, his army collapsed before he was captured and executed.

It was a brilliantly prepared campaign that changed the dynamics between the two powers, curtailed Shu-Han's long strategy and exploited Guan Yu being distracted with the Shu-Han army committed elsewhere while the attack was going on. Indeed encouraging it via diplomacy that flattered Guan Yu's ego, the choice of rank for the successor indicating no ambitions in that area, the retirement via ill-health to reassure Guan Yu his flank was safe. Armies like Sun He used to prevent possible interference while Lu Meng attacked but it relied on seizing the warning systems and then the two main garrisons surrendering thanks to discontent and the golden tongue of Yu Fan. The attackers for parts of a province were facing three points of defence even with Guan Yu away from Jing.

A surprise invasion gives the attackers an advantage but first to invade is first to win is not a rule I'm aware of. An army has to march at the speed of a lot of people, if bringing supplies with them then the baggage train will slow them and if living off the land, slowed by the need to raid and seize what they can off the land. If they come across a garrison that refuses to surrender, it is a risk to leave them there so the army may have to stop or leave troops behind to keep the garrison bottled up. To even start the attack might require a slow crossing of natural barriers like a deep river or mountains.

Meanwhile, via signals, riders or even runners, the message of the invading army gets sent down the line from people who don't have to wait for others, can refresh on friendly turf then continue on. Depending on the system of that nation, regional authorities may use the system to raise troops in the hopes of at least delaying the invaders until reinforcements can arrive. The defenders know the terrain and the area, where might be a good place to fight and delay to make it difficult, likely some sort of walls and fortified positions that are made to be able to delay an army for some time. At the capital, the ruler can begin raising troops and as well as existing troops, go themselves or a trusted general (or order troops moved from elsewhere) with said troops and gather more along the way if need be. Or they may decide a deny battle and lure in, drawing the attackers in with limited supplies to feed on, stretching their lines and at risk of encirclement on the terrain of their choosing.

Now an attacker's goal may well be to only be there for a limited time, to avoid getting bogged down and raid what they can. To then withdraw before reinforcements can come, annoying for defenders and then come again. It isn't impossible to stop an individual raid depending on resources and defenders may change their strategy overtime to give them a more mobile response to deal with the problem. However, a full-on invasion should probably expect significant resistance.