Visits and summits
Meeting in person builds trust like nothing else.
Invite a leader to visit you, or offer to visit them. Both leaders must be at home. If they agree, the traveller sets out with an escort.
A meeting that happens builds trust and goodwill, and the news reports it. Not turning up is an insult.
A leader who has never met you may want a visit before agreeing to an alliance.
The news reports a visit as soon as it is agreed, and a meeting once it is held. Your enemies then know where your leader is going, but not exactly where they are. An enemy at war with you may wait for them along the way, or watch the way from the air. The trip home is risky too.