The world’s wealthiest (eg, the American middle class) have the largest carbon footprint, and would make the most immediate impact by eating less meat.
The United States no longer has the largest total national carbon footprint. That title now goes to China, which has exploded past the US. The total carbon emissions title, since 1970 (roughly half, probably somewhat more, of the total since the Industrial Revolution, given ~30-ish year doubling rates), remains the US, though China are on track to surpass this within a few years.
Per capita, US residents still have a larger carbon footprint than China.
It's not the largest in the world, though, with that title going, depending on how you allocate, to petroleum exporting regions (e.g., Saudi Arabia), fueling and bunkering centres (e.g., Gibralter), ammonia fertiliser production (e.g., Trinidad and Tobago), or, if you're looking at direct local consumption rather than exports, largely Scandinavian countries with very large heating requirements.
Emissions by country, graph, 1970-present: https://en.m.wikipedia.org/wiki/List_of_countries_by_carbon_...
2017 national vs. per-capita emissions: https://en.m.wikipedia.org/wiki/List_of_countries_by_carbon_...
Cumulative emissions, 1970-2017: https://en.m.wikipedia.org/wiki/List_of_countries_by_carbon_...
The US agricultural system has global implications — from tooling and process, to subsidies and what poor countries grow. A fundamental change in the American diet would have an enormous ripple effect across the globe.
I try to do whatever lifestyle changes I can, but I also use Project Wren (https://projectwren.com/) to offset the remainder of my carbon footprint!