A long-standing precept of diplomacy has been that nations and their representatives do not intervene directly in the internal politics of other nations. It is an approach based on the long-established principle of state sovereignty and self-determination.
This now appears to be coming to an end, driven largely by a US President whose imperial sense of self and the promotion of US exceptionalism has seen protocol ignored and relationships disrupted.
The best illustration of this is a recent radio interview in which President Trump waded into Britain’s current election campaign, praising the UK’s Prime Minister, Boris Johnson, directly attacking the leader of the opposition, and calling for the Conservative and Brexit parties to unite as an “unstoppable force” to get a hard Brexit done so as to agree a beneficial trade deal with the US.