If you wish to inspect the front lines of the conflict between Donald Trump and California, head for San Diego.

Yes, so far the Golden State’s fight against the president has taken place in the courts and in cyberspace. And, sure, challenging The Donald’s legitimacy is not a mere local pastime but an all-consuming statewide prizefight. But as a matter of geography, culture and economy, the California-Trump war feels more intimate and higher-stakes in greater San Diego.

What makes Trump’s wild swings land as punches in San Diego? Because the city is more deeply tied to the United States than California’s other big regions. Los Angeles and the Bay Area define themselves as global mega-regions that deviate from national norms. But San Diego is unapologetically American — “America’s Finest City” being its most durable slogan. And because San Diego is the nation’s biggest border city and home to one of the world’s heaviest concentrations of military power, many San Diegans represent America not just as civic commitment but as professional duty.

But San Diego practices an open-hearted brand of Americanness that doesn’t mesh well with Trumpian isolationism. The population of active military and veterans leavens its patriotism with hard-won wisdom about the world. And San Diego institutions have built ever-closer ties to Mexico, encouraging cross-border commutes, contemplating a cross-border Olympic bid, and constructing an airport terminal that spans the border with Tijuana’s airport.

And so Trump’s obsession with border security is a threat to San Diego’s daily routine. The president’s increased deportations have inspired fear among immigrants in the city, and his blasts against cities that don’t use police to enforce immigration laws (San Diego among them) have inspired a poisonous backlash against federal law enforcement.

Further inflaming tensions, the Trump administration has made San Diego the host of the contest to design the president’s promised border wall. Sometime this month, contestant firms are expected to build samples of their border wall designs in the Otay Mesa neighborhood, which could touch off enormous protests.

Trump’s bigoted smear of refugees as security threats doesn’t go down well in the region, long a California leader in welcoming refugees. San Diego County has taken in roughly half of all the Syrian refugees who have settled in California.

Trump’s antitrade policies are a challenge for the border region’s economy and his attempts to restrict travel are seen as a danger to the huge tourism industry, which attracts more than 34 million visitors a year. His campaign against environmental regulation and proposed cuts to health and science funding threaten San Diego’s many top research institutes (studying everything from climate change to Alzheimer’s disease) and its life science companies.

The litigious Trump should stay out of San Diego courts. He made few friends in the region’s legal community last year with his racist attacks on U.S. District Court Judge Gonzalo Curiel, a San Diegan who is a highly respected member of the legal community there, for decisions in litigation against Trump University. Curiel is now assigned to a potentially precedent-setting case challenging the Trump administration’s deportation of a 23-year-old man who appeared to be legally in the United States.

For all of Trump’s provocations, opposing a president doesn’t come all that naturally to San Diego, a military town accustomed to saluting the flag and superior officers. Trump also has nodded in San Diego’s direction by proposing big increases in defense spending that could be a boon to the military and the area’s many defense-related businesses.

And San Diego’s middle-of-the-road politics — the county’s voters are closely split between Democrats and Republicans — makes dealing with Trump trickier than it is in California’s other, monolithically Democratic coastal cities.

So Trump poses a quandary for San Diego’s popular mayor, Kevin Faulconer, a Republican considered a possible contender for governor next year. Faulconer is not a Trump supporter, in word or deed — he backs comprehensive immigration reform, speaks Spanish, promotes cross-border trade, and touts his city’s Climate Action Plan. But his ambitions will require him to win the support of the small but powerful Republican base of voters who still mostly support Trump. Faulconer and other local Republicans, especially Rep. Darrell Issa of Vista (San Diego County), have had to walk a fine line between respecting a president of their party and criticizing Trump’s many affronts to decency and common sense.

The California-Trump war is still young. It’s possible that the president and his self-destructive tendencies will make it easier for San Diego to unite and oppose him with full force. But if the president hangs in and gains traction on his border, immigration, trade and budget policies, while delivering more money to the region’s defense industry, then steel yourself, San Diego, for very bitter fights in your very beautiful city.

Joe Mathews writes the Connecting California column for Zócalo Public Square. Twitter: @Joemmathews. To comment, submit your letter to the editor at http://bit.ly/SFChronicleletters.