In his sights were Canadian Prime Minister Justin Trudeau and French President Emmanuel Macron.
Both had criticised his plans to impose punitive tariffs on steel and aluminium exports to the US.
Both were name-checked in his eve-of-summit Twitter fighting talk, where he stuck two fingers up to those who thought this might be a chance to persuade him to change course.
“Look forward to seeing them tomorrow,” he wrote threateningly.
But the thing about Mr Trump’s personality-focused politics is that, if he’s attacking you, at least you know he’s listening.
Based on everything he’s uttered or tweeted before, during, and after this G7 summit, it seems when Mr Macron, Mr Trudeau or Mrs Merkel say something, the president of the United States takes note.
That’s not to say he’ll act upon their concerns – this G7 has shown he’s not easily moved – but he has at least heard them.
It’s not at all clear that is the case for Theresa May.
One of the prime minister’s goals for this summit was to formalise and build on the international solidarity in the wake of the Salisbury poisonings, which saw countries around the world impose sanctions on Russia and expel diplomats.
Mrs May did secure an agreement with all seven member states that they would work together to better coordinate responses to what she described as “malign state interference” in G7 democracies, but if she had hoped this might lead to a united stance that left Russia in no doubt about its isolation, she was to be disappointed.
Rather than turning the screw on Russian President Vladimir Putin, Mr Trump proposed welcoming him back to the G7 fold – a view supported by Italy’s new populist prime minister.
Mrs May attempted to remind the president that the reason the G8 became the G7 in 2014 was Mr Putin’s illegal annexation of Crimea. But this fell on deaf ears, with the president saying Russia had been booted out because of “something that happened a while ago”, and arguing the country could be an “asset” if it returned to the group.
As he left the summit, Mr Trump commented on how his relationships were faring with other leaders, saying of Mrs Merkel, Mr Macron and Mr Trudeau: “We have a great relationship. Angela and Emmanuel and Justin – I would say the relationship is a 10”. Of Theresa May there was no mention.
Neither did Mrs May have a formal one-on-one meeting with the president during the summit, despite holding such meetings with each of the other leaders present.
Downing Street was keen to play this down, arguing the prime minister has spoken to Mr Trump recently on the phone and they have a good relationship but suggesting formal bilateral meetings were just not the way Mr Trump does business.
That’s not really true. Mr Trump had bilateral meetings with Emmanuel Macron and Justin Trudeau.
Asked twice by Sky News whether she had requested such a meeting, the prime minister refused to say – an odd response if you had concluded that informal words in the sidelines would suffice.
The uncomfortable reality for Number 10 is that Mr Trump is selective about who he sits down with. At this G7, Mrs May was seemingly not a priority.
Even in her concluding comments, the most Mrs May could say was that she’d had “brief words” with the president in which he said he was looking forward to his upcoming visit to the UK.
It’s something, for sure, but considering the seriousness of issues being discussed here, the prime minister surely would have hoped for more.
In Trump’s politics of the personal, failure to get attention is a problem if you want to have any chance of influencing him – special relationships in his world are about individuals, not historic convention.
So what could Theresa May do differently?
No one would expect her to offer a handshake so firm it squeezed the blood out of Mr Trump’s hand, as Mr Macron appeared to do, but she doesn’t appear to have her own alternative means of applying pressure in the Trumpian diplomatic paradigm in which she finds herself.
Some of that might simply be down to charisma and showmanship, not traits Mrs May has ever suggested are her political strengths.
But political visibility is not just about how you hold yourself, your eye for a photo opportunity or your propensity for a witty quip. Weakness can sometimes just be a political fact.
Whether they are aware of the ins and outs of the infighting within her cabinet, be it David Davis’s recent threat to resign, or Boris Johnson’s leaked criticisms of her leadership, the overall mood music around Mrs May’s grip on power is something the other G7 leaders are more than aware of.
The whole point of the G7 is that it provides a select forum for long-term decision-making, coordination and global leadership.
But the election a year ago severely reduced Mrs May’s ability to act decisively and left a persistent question mark hanging over her political future.
Mr Trump, in particular, admires power. He pays attention to it. He feels challenged by it and cannot resist engaging with it.
Yet for all her experience, work ethic and commitment – the strengths her supporters point to – straight forward, uncompromising political power is something Mrs May lost on 9 June 2017.
A seat at the top table means very little if the person sitting there is barely heard.