During ancient times, when one king attacked another hundreds of miles away from his own kingdom, what stopped the neighboring ones from attacking his own kingdom, which would presumably be vulnerable due to most of the army away fighting a war?

The Ancient Tamils believed that the Five Arms of a good King were the Armies, the Treasuries, the Ministers- and the Ambassadors and Spies.

A Late Classical Playwright would mock them as the Five Plagues of Kings but that's a story for another day.

Anyway…

The Shastras advocate for Dharmayuddha and Dharmavijaya to be accomplished by a properly consecrated Chakravartin who is to uphold Dharma with the help of the populace of all castes and his brother kingly vassals. However, the Shastras also recognised that all morals will be lost in the Kali Yuga and men will grow cruel, lazy, ingratiate, greedy, faithless, the lands and people will splinter into pieces, and Dharma will be lost.

In response to this, the great thinkers of Bharata proposed several ideas to be adopted by any far-sighted lord who wished to keep his realm secure. Among them were the four maxims of Chanakya- “Saam, Dham, Dand, Bhed” or Rewards, Threats, Division, and Punishment.

To such utopian ideals, other thinkers such as Raja Bhoja of the Paramara line of Malwa have countered with the more realistic “Dand, Bhed, Dand, Bhed”- Punishment, Division, Punishment, Division.

Note that neither include the terms “Sabka, “Saath” or “Vikas”.

Now the major Shastras are at pains to outline the nature and duties of spies to be used by the sensible lords for “men see through eyes, cows by smell, brahmins by learning, and kings by spies.” Do note that while the Ancient and early Classical Tamils advocated rights for diplomats, their more Northern counterparts had little love or respect for these well-spoken thugs, according them to be merely yet another rather boisterous malcontent, albeit ones not to be given the beatings they rightfully deserve for their constant lies.

These spies and ambassadors are to subvert border defences, bribe merchants and ministers, raise unrest among the rival populace by means of fraudulent prophecies & art, spread untruths among the opposing armies, and encourage filial strife among opposing families. Furthermore, armies are unwieldy and require massive amounts of gold to raise and maintain; proper relations with guildsheads and malcontent vassals can aid any enterprising King against his rivals. The flow of gold determines the hiring of mercenaries; the harvest of crops harkens the shriek of war conches. Skanda is the patron of Warlike hosts and Cunning thieves alike.

At all points, the King must be conscious of the concept of Circle of States.

Here we might point to the example of the Pallava lords Nanakkasa and Santivarman- who served their gods, king, and the Tamil people- by not only frustrating the imperial ambitions of the Highking of the Karnatas, Krishna II of the Kadambas, possibly looking to exploit the decline of Gupta power- but also winning over their Ganga vassals as well as setting up a puppet Kadamba pretender through the branch house of Vishnuvarman.

The King Vigraharajadeva of Sakambari, by means of his prowess in such Kutayuddha, kept his powerful rivals flummoxed while he expanded his territories and acquired great might. Both Rudradeva, who struggled the fortunes of his Kakatiya line afloat, in the aftermath of the collapse of the Rashtrakuta Emperors and Lalitaditya of Kashmira, Emperor in his own right, utilised assassins to weaken rivals on the eve of their campaigns.

Simultaneously, punishments of the most cruel and infamous nature just be adopted against such actions performed against oneself. The Manimekhalai speaks of a case in Kalinga where Vasu, lord of Simhapura, had a certain guildsman named Sangama beheaded under the auspices of the officer Bharata. Similarly, the Shastras recommend that blinding be a fitting Punishment for rabble-rousers and propagandists. Fear of oneself weakens the resolve of rivals; Reputation for steadfastness in Dharma encourages doubts among the enemies and rivals. Tales of one's brutality make the enemies' women call for for peace and the rivals' recommend for caution.

Fear and brutality was recommended, for this is Kali Yuga; Bharatas live in terrible times and thus it is recommended that we must be terrible ourselves. Else the end is certain.

A more recent and pertinent example is that of the noted Nazi and Fascist Anangabhimadeva III Chodaganga, Lord of the Three Kalingas, who upon being told that the Peacefuls he'd crushed were saddened at not going to their Heaven on account of failing to slaughter Oriya kaffirs like us, expressed concern over their immortal souls and had them all impaled alive on tall poles so that their alleged angels would have an easier time taking them upstairs. On account of this, he lived a long and happy life, and his line held fast and true to the worship of the true Lord of the Universe for centuries after him.