A huge portfolio of issues – the very matters that define a civil society and that have been at the heart of the American experience, from the country’s 18th-century revolutionary beginnings to its 21st-century tensions – now are on full display, and are subject to full tests.

Instantly, from coast to coast, from the marble-and-sandstone buildings of political Washington to the migrant neighbourhoods of a country convulsed in debate over fundamental principles, a confrontation between protesters and military personnel thrust a consequential set of fundamental matters into contemporary, contentious attention:

The country’s tradition of welcome for the tired, the poor and the fleeing. The pre-American Revolution heritage of protest and rebellion. The Constitution-era enshrinement of protections for free association and protest. Decades of pre-Civil War contention over the rights of states versus the prerogatives of the central government. Centuries of struggle over the limits to presidential power.

And, lest we forget in the bursts of tear gas and rubber bullets in Los Angeles, the right of a government to enforce its boundaries and its laws.

In a presidential order and a confrontation on the streets of a sprawling city where 154 languages are spoken in the public schools, all these issues abruptly have come into sharp focus in California, known for its clement climate and laid-back insouciance.

But in the brief course of an angry weekend, a state which since the Second World War was regarded as the harbinger of the future suddenly was transformed into a de-silvering mirror of the past.

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The questions that Americans are wrestling over – those on the right and on the left, on the MAGA vanguard and on the civil-liberties ramparts, and on the Washington political stage and in the California streets – are as old as the country and as fresh as the past hour’s social-media feed.

They have new relevance in a new period of American political struggle.

Seldom – not during the Watergate upheaval, not during the civil-rights era, not during the progressive period of the Republican Theodore Roosevelt and the Democrat Woodrow Wilson, not during the innovations of the New Deal or the recalibrations of the new conservatism of Ronald Reagan – have so many issues, so fundamental to the country’s heritage and identity, been at play.

This is but the latest episode of the clash of titanic principles.

The most recent staging ground was a protest over the Donald Trump administration’s determination to round up, punish, and deport migrants whom the President described on his Truth Social platform as “violent, insurrectionist mobs” and ordered his administration to “liberate Los Angeles from the Migrant Invasion.”

In a dramatic example of politics being a matter of, as a New Deal aphorism put it, where you stand depends on where you sit, progressive and advocates of migrant rights mounted the palisades of media to declare their outrage while Trump supporters stoutly mounted a law-and-order defence.

But among the cries from both sides, and amid the visual images of conflict, the difficult complexities of the moment were revealed.

Though the direct provocation was not clear, Mr. Trump called out – essentially nationalized – California’s state-based military force. The administration’s defence of the action came in a Pentagon memo that cited “violent protests [that] threaten the security of and significant damage to Federal immigration detention facilities and other Federal property,” adding, “To the extent that protests or acts of violence directly inhibit the execution of the laws, they constitute a form of rebellion against the authority of the Government of the United States.”

The President did not send in regular military troops.

“The issue here is mostly the stupid language Trump uses, because he is interested in making things worse,” said Bruce Ledewitz of Duquesne University’s Kline Law School. “A lot of people are here illegally and thus are subject to deportation. When you have violent rioting and people are interfering with law enforcement, you call out the National Guard.”

Indeed, military forces have been used in ways that progressives supported and, in retrospect, revere – especially when John F. Kennedy and Lyndon Johnson employed them in the civil-rights era, which Mr. Kennedy said raised issues “as old as the scriptures and as clear as the American Constitution.” Republicans such as Dwight Eisenhower (during the 1957 Little Rock, Ark., desegregation struggle) and George H.W. Bush (during the 1992 riots following the beating of Rodney King) also have summoned troops at times of contention.

One of the issues here is the deployment of the National Guard over the objections of California’s governor, Gavin Newsom, with whom Mr. Trump repeatedly has sparred as the personification of blue-state progressivism.

In taking this action under the Armed Forces Act, the President also escalated a long-simmering dispute with Mr. Newsom that flared recently over issues involving transgender athletes in women’s sports. The President has threatened to punish the state by withdrawing federal funds much the way he has frozen US$2.2-billion in federal grants to Harvard University in a separate, unrelated matter of contention.

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Stephen Miller, the White House deputy chief of staff regarded as one of the most ardent administration advocates of migrant deportations, posted on social media that the struggle on the streets of Los Angeles was “a fight to save civilization.” The opponents of the action felt precisely the same way.

But this is not a garden-variety question of differing opinions.

Both left and right recognize this as a period when a series of foundational questions are being weighed, including the limits of presidential power – and both Trump supporters and opponents see the Los Angeles episode as a test, even one he has sought, of the President’s inclination and desire to extend his muscular use of executive powers in new directions.

An 1878 law known as the Posse Comitatus Act implicitly permits the deployment of troops in some discrete domestic situations, but specifically bars it from direct law enforcement. In coming days, politicians and scholars are almost certain to debate the relevance and application of this law to the current situation and the current administration.

“We’re going to have troops everywhere,” Mr. Trump said Sunday. He was asked about the bar that must be passed for deploying the Marines. “The bar,” he said, “is what I think it is.”