The simple answer is that those bases aren't considered US territory and do not constitute annexation, and in most cases are there by invitation, such as to defend against the other, worse, hegemon.
If those bases aren't considered US territory whose territory are they?
In case of United States Army installations in Germany:
"
The legal status of Americans stationed in Germany is governed by special international agreements including the NATO Status of Forces Agreement.
People stationed at overseas bases or posts must remember German laws apply to them, too. To protect yourself, know what rights you have as an American. Whether German authorities exercise their right of jurisdiction depends on the circumstances of the case.
"
If those bases aren't considered US territory whose territory are they?
In case of United States Army installations in Germany:
" The legal status of Americans stationed in Germany is governed by special international agreements including the NATO Status of Forces Agreement.
People stationed at overseas bases or posts must remember German laws apply to them, too. To protect yourself, know what rights you have as an American. Whether German authorities exercise their right of jurisdiction depends on the circumstances of the case. "
https://www.ramstein.af.mil/About/Fact-Sheets/Display/Articl...
In case of Guantanamo Bay Naval Base:
"At the United Nations Human Rights Council in 2013, Cuba's Foreign Minister demanded the U.S. return the base to Cuba."
https://en.wikipedia.org/wiki/Guantanamo_Bay_Naval_Base#Cold...