The nub of this war is whether there was an understanding
(not a written contract) if the USSR agreed to EastGermany
on unification of Germany joining NATO that NATO would not
move "one inch" eastward after that.
IMO that agreement was understood/accepted by both parties then
and hence no need for a formal (written} agreement . Had there been
dispute then , IMO there would be a written agreement/disagreement
to that extent .
The context of the understandings are documented below by Wikipedia as follows
with the bottom line being "if you cant take a country on its word then why
should accept its written agreement"? According to Wiipedia policy, if others
can refute this by providing authorative documentation then Wikipedia
will gladly amend this report.
(source: Wikipedia)