When the Buddha had outlined the 7 principles of non-decline to Vassakāra, he responded that the Vajjians could indeed not be overcome except by upalāpana or by “sowing dissension”. The sense of upalāpana has not been clearly recognized by translators. Bodhi has “treachery”; Walshe has “propaganda”; Brahmali (in different contexts) has “befriends”, “supports”, “wins over”; Thanissaro has “befriends”; I had “diplomacy” as does Ānandajoti.
As for dictionaries, Digital Pāḷi Dictionary has “persuading; coaxing; cajoling; buttering up”, to which Cone’s dictionary adds the sense “wins over”.
Let’s have a look at some contexts. Here are the relevant Vinaya passages in @Brahmali’s translation.
pli-tv-bu-vb-pj1:6.1: Sometime later, in the Great Wood near Vesālī, a certain monk befriended a female monkey by giving her food. He then had sexual intercourse with her.
(You might be thinking that the exact meaning of upalāpana is the least interesting thing about this passage, but I will ignore that and forge ahead!)
pli-tv-bu-vb-pc70:1.55: If a monk supports that novice monastic, or he is attended on by him, lives with him, or shares a sleeping place with him, even though he knows that he has been expelled in this way, he commits an offense entailing confession.’”
…
Supports:
if he supports him, saying, “I’ll give him a bowl,” “I’ll give him a robe,” “I’ll recite to him,” or “I’ll test him,” he commits an offense entailing confession.
pli-tv-kd2:21.2.4: Those monks should treat that learned monk with kindness. They should assist him and support him, and they should attend on him with bath powder, soap, tooth cleaners, and water for rinsing the mouth.
pli-tv-bu-vb-pj2:7.26.18: A man befriended that monk by giving him food. Seeing a customs station, he gave the monk a valuable gem, saying, “Sir, please take this gem past the customs,”
pli-tv-bu-vb-pj4:1.3.16: Or a bad monk takes valuable goods and requisites from the Sangha—a monastery, the land of a monastery, a dwelling, the site of a dwelling, a bed, a bench, a mattress, a pillow, a metal pot, a metal jar, a metal bucket, a metal bowl, a machete, a hatchet, an ax, a spade, a chisel, a creeper, bamboo, reed, grass, clay, wooden goods, earthenware goods—and uses them to win over and create a following among householders.
In the suttas we find:
sn3.25:4.13: In this royal court there is abundant gold coin and bullion stored in dungeons and towers. Using this wealth we can pay off an approaching enemy.
So in all the cases we find the sense of using some kind of material gift in order to win someone over to your side. This might be done in a wholesome way, as for example providing suitable gifts for a senior monk, but in most cases it is a form of manipulating or bribing. In any case we can definitely rule out “treachery”, “propaganda”, or “diplomacy”. “Supporting” and “befriending” are not completely wrong, but I feel there could be a clearer rendering.
The sense from SN 3.25 is particularly relevant, as it shows exactly how a king might “pay off” or “bribe” an enemy.