If that were the case the US (or any other democracy) wouldn't be able to sustain so many security parternships and alliances, which has clearly not been the case.
American strategists, more than anyone, understands the fickle nature of democracies, which is why they have mounted so many coups world wide.
To accept the 'largess' of US military protection is a double edged sword in that you also need to allow the US to base military personnel on your soil, as well as give their intelligence people effective free rein to do as they like in your territory.
It is a shackle as much as a shield.
The moment anyone steps out of line of what the US wants them to do is the moment they find themselves on the receiving end of the full might of the US state subversion machine.
It will start with the unleashing of US controlled western media attack dogs, which are usually enough to cripple candidates in democracies.
If that fails, you can quickly find US proxy foot soldiers out in force protesting. With social media bots whipping up popular anger with fake news and faker outrage until the whole thing becomes a self-sustaining rage machine.
If you are strong, patient and smart, you can weather that out by exhausting the western foot soldiers and wait for populate oppinion to turn against them (Hong Kong occupy).
If your government is not united, someone might cave and either mount a soft coup and remove the person the US find most offensive (Egypt); or a hard coup to put someone acceptable to the US in charge if elections failed to do that (Egypt again under Sisi).
If your government is strong by the US intelligence machine has a strong presence on the ground in your country. They may well 'help' things along by escalating the violence to force loyalists to turn against their leaders (Ukraine).
If the US doesn't have the field presence to effect regime change by stealth. They can either use radicals to attack the security forces and then use any resulting crack down as a lever to push a country into full blown civil war (Libyia and Syria). And/or use same said radicals to launch false flag operations to create a pretext for them to enter the conflict directly (Libya and Syria).
If you think these underhanded tactics are only used on 'enemies', you only need to look to Turkey, a full fledged NATO member to see how wrong that assumption would be.
I have little doubt that if a South Korean president was seriously considering ditching the US and wanted to host Chinese military bases instead, he/she would either fall from an US black site launched 'popular' protest movement, or find South Korean troops and tanks smashing down his/her door.
The US does not have security 'partnerships' (the sole exception may be Israel, but that's a very special case), it has countries it is still effectively occupying militarily (Germany, Japan and South Korea) and it has protectorates (the rest of NATO).
It tries to play nice and make everyone choose to side with the US by their own free choice, but is ready, able and willing to use any and all of the methods above and more to 'set straight' any leader or supposedly allied 'partner' who dares to do anything that would significantly threaten US interests and plans.
That is the real gaurantee holding US alliances together. And everyone knows it to a certain extent, even if subconsciously.
Many a times, I have heard interviews where former senior British military and political leaders have plainly stated that when the Americans go into full superpower hulk mode, they know they need to either get onboard or get the hell out of the way.
That is not a relationship of partners, but rather of master and pet. And that's the only kind of relationship the US would tolerate with its allies.
