How to...

excuse me, and everything. could someone please take the time to help me?
how do you setup offensive military bases?
by land?
by air?
i'd really appreciate help

Offensive military bases and many more such items can be bought on the market.

go to the top-of-the-page menu.
click on Trade --> World Market Trading --> Products in stock Buy and Sell.

you get a long list of products you can purchase.

You can also read the game docs for once

i did...i ment
how do you make one, at another country

you setup and airbase construction unit and airlift it and then you make it build an airstrip theres also remote depots that you can setup in another country with permission store weapons there for any units that you airlift into the country