A second camp, representing the emerging left within the Democratic Party and including the likes of Senator Bernie Sanders and Representative Ilhan Omar, is articulating more fundamental concerns about American support for the Saudi kingdom. Their goal is a substantial change in policy, not preservation of the underlying relationship. As a younger, more diverse generation of voters emerges to challenge conventional politics, such a prospect cannot be ruled out.

Though the strands weave together into impressive dissent — the Trump White House has been forced to rely on executive orders to override it — overall support for the kingdom in Washington endures for now. But that cannot be taken for granted.

If the world finally gets serious about tackling the climate emergency, a large proportion of existing oil reserves will have to remain in the ground, leaving the Saudis sitting on stranded assets. So long as oil is the lifeblood of the world economy, strategic control over the key reserves of the Gulf is a major source of power in the global system. The petrodollar wealth generated by oil sales also constitutes a lucrative source of investments and arms deals. But if the world decarbonizes, it is difficult to see why continued support for an authoritarian state of diminishing wealth and relevance would be regarded in Washington as worth the political cost. As with much else, climate change could upend foreign policy.

Similar dynamics are unfolding in Britain, Riyadh’s other major Western ally. Last week’s court judgment did not cut off all arms transfers and is subject to government appeal. But it has materially impeded those exports for now, which is significant given the importance of British-built aircraft in the coalition’s bombing campaign and the centrality of arms sales to Britain’s relationship with Saudi Arabia.

More significantly still, the British Labour Party is undergoing a painful but definite process of change, mirroring that working its way through the Democratic Party. The leadership is consistently critical of the regime’s human rights record and its conduct in Yemen, and has sidelined pro-Saudi Labour legislators. That leadership is buttressed by an empowered and expanded party membership, also committed to a new direction in British foreign policy.

The strategic bonds between the Atlantic powers and the House of Saud have survived many crises over the years, and for all we know they may survive for a second century. But the existential threats are now plain to see, and if anyone in Riyadh, Washington or London has a serious plan to preserve the status quo, they are keeping it a closely guarded secret.

Recent Saudi efforts to stay close to Mr. Trump’s White House have been undeniably successful. However, the kingdom may come to regret betting everything on a presidency that appears more representative of the uglier aspects of America’s past than its future. In the coming years, the regime risks finding itself running out of cash, out of friends and out of time.

David Wearing (@davidwearing) is a specialist on British-Gulf relations and the author of “AngloArabia: Why Gulf Wealth Matters to Britain.”

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