Years ago I wrote just for myself a list of restrictions for national security-related foreign policy that should be observed in order to prevent the repetition of severe mistakes. I wrote it in the style of 22 constitution articles (just for fun).
It was interesting to re-read this old document; I felt almost no need for changes.
One of these rules was that no guarantees of sovereignty should be given except to states which were demilitarised.
Why should any nation feel that it's a good idea to guarantee another nation's sovereignty if said other nation isn't ready to return the favour (thus establishing a proper, formal defensive alliance)?
One motivation that I can think of is the intent to use this to become involved in a conflict. That's what warmongers do.
Another motivation could be a missionary desire to establish world peace by deterring potential aggressors. On the other hand - why not ratify a real alliance in this case? The guarantee of sovereignty already includes all disadvantages of a defensive alliance.
Número tres finally could be a guarantee of sovereignty to a state that's not really threatened by a third nation, but needs to be convinced that ours is no threat. Italy can easily guarantee the sovereignty of San Marino and South Africa can easily guarantee the sovereignty of Lesotho without any disadvantage, for example.
Such guarantees of independence are in fact unidirectional alliances; one power pledges to defend another without getting the other's pledge in return. This does usually not make sense in a national security context for it could draw the protecting country into an otherwise avoidable conflict, but it makes a lot of sense in a "let's meddle in distant regions' affairs" policy.
In other words: I'd never advise to offer guarantee of sovereignty to another state except in rare and very special circumstances. One such exception would be if a small country saves the expense of a military which would be ineffective anyway and becomes demilitarised. It makes sense for France to protect Monaco and Andorra, for Italy to protect San Marino, for the U.S. to protect Costa Rica. Many small countries would be well-advised to maintain combat proficiency only in a Gendarmerie.
PS: Part I