“乞盟”的英文翻譯可以是 **"to beg for an alliance"** 或 **"to sue for a treaty"**。

具體選擇取決于上下文：

1. **"to beg for an alliance"**
強調在弱勢或戰敗時，懇求與對方結盟或求和。

2. **"to sue for a treaty"**
更正式，特指通過外交手段請求締結條約（尤其是停戰或和平條約）。

**例句參考：**
- 古代戰敗國常被迫乞盟。
*Defeated states in ancient times were often forced to sue for a treaty.*
- 他們在軍事失利后不得不向鄰國乞盟。
*They had to beg the neighboring country for an alliance after their military defeat.*

如果需要更文學化或歷史語境的說法，也可用 **"to implore for a covenant"**（懇求盟約）。請根據具體語境選擇最貼切的譯法。