G7's Biarritz meet will be heated like the 2018 Canadian summit

G7 presidents and prime ministers meet in Biarritz, France, this weekend and it may not just be the weather that proves hot. On a range of issues, further disagreements are likely between the western powers with US President Donald Trump expected to raise the diplomatic temperature.

The context for this weekend's session is last year's Canadian summit, where there was an unprecedented failure to agree to an end-of-summit communiqué. And this was given added spice by remarkable undiplomatic language, including Mr Trump's characterisation of Canadian Prime Minister Justin Trudeau as "so indignant"; Mr Trudeau in turn called the US president's trade tariffs "laughable".

The personal animosity on display between Mr Trump, who left early to visit Kim Jong-Un in Singapore, and other leaders provided atmospherics for wider policy splits. The US leader, for instance, called on the first day of the summit for Russia to be allowed to rejoin the group (as the G8), as was the case from 1997 to 2013, a call which he renewed on Tuesday this week.

Yet other G7 leaders called for a "rapid and unified" response to malign international interference, including by Moscow, such as cyber and chemical weapon attacks like those last year in Salisbury, England. Despite the US president's desire for warmer ties with Vladimir Putin, there is little sign that Russia will be invited back to the club any time soon after being told it can rejoin only if "it changes course and an environment is once again created in which it is possible for the G8 to hold reasonable discussions".

Yet, amid continuing divisions from trade to climate change, which gave to talk last year of a "G6 plus 1", there may be cause for upside surprise this year, given the changing dynamics of the leadership. For instance, Boris Johnson joins the meeting for the first time and it is possible he could act as a bridge between Mr Trump and other G7 leaders.

His assumption of power, and the forthcoming Oct 31 Brexit deadlines, will mean the UK's exit from the EU will be unofficially on the agenda among London, Brussels, Berlin and Rome too. Normal diplomatic back-channelling between these powers has intensified before Biarritz, which could set the ground for summitry in Brussels in September or October.

Leadership transitions are underway not just in the United Kingdom, but also the EU and Italy too with Jean Claude Juncker, Donald Tusk and Giuseppe Conte stepping down. And potentially in the next 12 months too, this could be the case in Canada, with an election scheduled this autumn; over in Germany, Angela Merkel's long reign is in its twilight.

Looking further out to the second half of 2020, much may also depend upon whether Mr Trump is returned to power. While the fissures within the G7 did not begin with his election in 2016, they have been exacerbated by it.

Since the collapse of the Soviet Union, there have been a series of intra-Western disagreements over issues from the Middle East (including the Iraq War opposed in 2003 by France and Germany) through to the rise of China, over which some European powers and the United States disagree on the method of engagement with Beijing.

Yet, despite occasional discord, key Western nations generally continued to agree until the Trump presidency around a broad range of issues such as international trade, backing for a Middle Eastern peace process between Israel and the Palestinians along Oslo principles, plus strong support for the international rules-based system and the supranational organisations that make this work.

Yet today, more of these key principles are being disrupted if not outright undermined by Mr Trump's agenda - so much so that the French hosts are reportedly considering whether the traditional end-of-summit communiqué will even be issued.

Emmanuel Macron reportedly does not want to adopt formal conclusions if rancorous policy differences do re-emerge with Mr Trump.

Take the example of international trade, which ended with the US leader being isolated last year, following "unanimous concern and disappointment" of Canada, United Kingdom, Japan, France, Germany and Italy. Trade will again be an undercurrent of tension in Biarritz, with Washington considering imposing in November new tariffs on European car imports.

To plaster over these cracks, it is therefore possible that significant emphasis could be put at the meeting on finding greater G7 consensus on a range of security and geopolitical issues. Potential examples here could include North Korea, plus the continuing clampdown against terrorism.

There is also possibility of a further G7 statement over its concerns over Venezuela. Western leaders remain worried about the destabilising political effects of that situation for the wider region, including bordering states of Brazil and Colombia, and the fact that Venezuela has the world's largest proven oil reserves.

This would, yet again, highlight the group's often under-appreciated importance as an international security lynchpin - despite the fact that it was originally conceived in the 1970s to monitor developments in the world economy and assess macroeconomic policies.

The 2017 G7 summit, for instance, was dominated by the aftermath of the Manchester terrorist attacks and development of a new G7 terrorism action plan, as well as the nuclear tensions on the Korean peninsula. Mr Trump also pushed then for other G7 members for Nato to become a full member of the global coalition against Daesh.

The G7's involvement in this multitude of geopolitical dialogues is not without controversy, given its original macroeconomic mandate. For instance, China strongly objected to discussion of maritime security in Asia at the 2016 Japan-hosted summit.

It is sometimes asserted, especially by developing countries, that the G7 lacks the legitimacy of the UN, or even the G20, to engage in these international security issues, and/or is a historical artefact given the rise of new powers such as China and India. However, it is not the case that the international security role of the G7 is new.

An early example of the lynchpin function the body has played here was in the 1970s and 1980s, when it helped coordinate Western strategy towards the then-Soviet Union. Moreover, following the September 2001 terrorist attacks, it assumed a key role in the US-led "campaign against terrorism".

Taken overall, Biarritz will see splits again, including on Russia and trade, although Mr Johnson's stance toward Mr Trump may be significantly warmer than other leaders. While some western fissures pre-date his presidency, his agenda has grown these gaps and next year's US election is by far the most important of the potential G7 leadership transitions this year and the next.