一亩三分地里 is often used to describe a confined space that is separated from outside world. One can focus on his/her own business in this space with no need to care about other things.

My humble try:

Russia is just his backyard and anything he decides others don't have a say.

In case you are not familiar with the cultural background for "一亩三分地". I think this phrase probably originated from 包产到户 (where the country contracted lands to farmers individually) or when people started to have "自留地”(where families got little pieces of lands around their houses to grow vegetables or alike for their own consumption). In either case, that “一亩三分地” were treated as personal property and the owners (though could actually be just contractors) got to make all decisions regarding what to grow and how, when... Remember this was after around 30 years of all the farmers in a village working together in the public-owned fields and every decision being made by the whole group or some sort of committees.

Please anyone go ahead correct me if my understanding of 一亩三分地 is wrong or if the background I gave is too off.


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Note added at 38 mins (2007-03-06 22:49:05 GMT)
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Another though: 一亩三分地 could just mean that the area is not big at all comparing to his power range, or in another word, it is totally under his control.

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Note added at 1 hr (2007-03-06 23:14:27 GMT)
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Thanks to Jian Zhang's reference (See peer comments below for the link):
人们常常用“一亩三分地”作为利己主义的代名词。那么，“一亩三分地”这句成语是怎么来的呢?

　　满族原是我国北方的游牧民族，1644 年建立清王朝之后，为了及时了解农时，熟悉节令，居住在深宫大院里的皇帝便在惊蛰时分乘龙车从正阳门到先农坛耕地。当时划出的这一块地叫“演耕田”，每年由皇帝、皇后演试“亲耕”，表示普天之下该种五谷了，并以此显示其对农业生产的重视。这种做法世代沿袭，他人不得将地改作它用。当时皇帝“亲耕”的这块地恰好为“一亩三分地”，于是，人们推而广之，将个人利益或个人势力范围称为“一亩三分地”。（作者张天雁）

So "一亩三分地” here just means Russia is totally under his control for his own befinit.

我觉得后院的中文意思来表达一亩三分地是很好的，但是我查了backyard的英文意思，并没有我们中文里包含的意味。shaunna能给出refrence吗，我也想弄清楚呢，谢谢 ：）

基于以上各位，我给出另外一个version，试翻如下

It means that in Russia, which is literally the territory that he has the supreme power, nobody could be in his way on any thing he decided.