Question: > 
> [ [Wiktionary:](https://en.wiktionary.org/wiki/impero#Latin) ]   Etymology
>    From [in-](https://en.wiktionary.org/wiki/in-#Latin-in) +‎ [parō](https://en.wiktionary.org/wiki/paro#Latin).
> 
> 
> 1. (with dative) I command, give orders to
> 2. I rule, govern
> 3. I demand, impose
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> 
> 

Because the 3 definitions above matches the meaning that one prepares (i.e. parat) TO or TOWARDS something, should not 'ad'- have been the apter prefix? 

How do the 3 definitions above match the meaning of preparing IN someone/something?

Answer: 
There actually is an [apparo](http://logeion.uchicago.edu/index.html#apparo) (ad + paro). The prefix ad- was assimilated and the 'd' turned into a 'p'. The word *apparo* means to prepare or provide. 

As C.M. Weimer commented, [in-](http://logeion.uchicago.edu/index.html#in) has an accusative sense which means *to* or *into*. There are many compound verbs prefixed by *in* which carry this sense, such as *ingredior*, *infero*, and *ineo*.

There is another explanation for the meaning of *impero*. Compound verbs often take a dative, and the meaning of this dative is often related to the prefix. Considero *impero tibi*, which means "I give a command to you". See how the preposition "to" in the English translation conveys the sense of the prefix? Another example would be *absum tibi*, "I am absent *from* you".

So, considering the accusative sense of *in-*, the meaning of impero is quite logical.
The cases are derived from the parts (and meaning) of the word, not vice versa. Furthermore "impero" can have accusative-like component as well, referring to what is being commanded, albeit this is typically a verb. So to distinguish, the target of the command is probably considered the "beneficiary" of the verb, hence dative.

As to why it is *im-* + *parare* that evolved into the meaning of commanding (and not *ad-* or something else), I can only say that historical semantics are pretty goofy sometimes and we have plenty of similar strange examples in Czech. The metaphor probably comes from *parare* meaning primarily "prepare", but also by extension "arrange", "contrive" etc. *Im-parare* may have originated putting this purpose to do something into somebody.