He guarantees that only his sister, the marquise, will be present, and he hopes that 'Ce bon Steinmetz,' will accompany you, and also the young lady, the cousin of the princess." Steinmetz threw the letter down on the table, left it there for a moment, and then, picking it up, he crossed the room and threw it into the fire.
"Which means," he explained, "that M.Vassili knows we are here, and unless we dine with him we shall be subjected to annoyance and delay on the frontier by a stupid--a singularly and suspiciously stupid--minor official.

If we refuse, Vassili will conclude that we are afraid of him.
Therefore we must accept.

Especially as Vassili has his weak points.

He loves a lord, 'Ce Vassili.' If you accept on some of that stationery I ordered for you with a colossal gold coronet, that will already be of some effect.

A chain is as strong as its weakest link.