Mexico border wall: Why is Donald Trump deploying the National Guard?

President orders show of force in response to frustrations over blockade's funding and apparent spike in illegal crossings

US President Donald Trump has called on the National Guard to defend the border with Mexico.

The order - in the form of a signed memorandum - follows months of frustration for the president as he struggles to fund his long-promised "big, beautiful wall" and follows his expressing concern over a "caravan" of 1,000 migrants from Central America crossing Mexico in pursuit of a better life in the US.

President Trump has warned that the problem of illegal immigration into the US from its southern neighbour is reaching a "crisis point" and threatened to withdraw foreign aid to Honduras - the country of origin of many of the convoy's participants.

The signed memo on Wednesday evening states that "the security of the United States is imperilled by a drastic surge of illegal activity on the southern border".

Citing his predecessors George W Bush and Barack Obama summoning the National Guard to the southern border in 2006 and 2012 respectively, Trump's memo reads: "The crisis at our southern border once again calls for the National Guard to help secure our border and protect our homeland."

Kirstjen Nielsen, Homeland Security Secretary, made the initial announcement on Wednesday afternoon but was unable to confirm the number of troops that would be dispatched, the cost of the mission or precisely where the guardsmen would be positioned but did say that an up-tick in attempted crossings over the last month was the reason for the call-up.

This last point contradicted assumptions that illegal immigration from Mexico was currently at a historic low, based on a Homeland Security report concluding precisely that published as recently as last September.

But why deploy the National Guard, a reserve military force comprised of troops with part-time jobs elsewhere, rather than the US Army itself?

Roughly equivalent to the Territorial Army in the UK, the short answer is that the National Guard is not subjected to the same restrictions as the professional military.

The 1878 Posse Comitatus Act, signed into law by President Rutherford Hayes after the Civil War, prohibits the federal use of American military might to enforce laws and domestic policies within US borders.

The Army therefore cannot perform civilian law enforcement tasks on US soil like arrests, apprehension, interrogation or detention without the act being suspended by Congress.

It could though play a secondary role in relation to the Mexican border wall, assisting with tasks like training, construction or intelligence gathering.

The act does not apply to the National Guard, however, which can carry out law enforcement duties as long as it has the support of the state in question's governor.

But making arrests or searching suspects without explicit congressional support would contravene the Guards' regulations, meaning that Homeland Security is likely to play them in a supporting capacity, assisting the Customs and Border Protection agents already patrolling the border.

Given that the National Guard is organised on a state-by-state basis, any federal decision to deploy them involves effectively assuming control of a local governor's resources - a move that can be contentious.

Arizona Governor Doug Ducey, however, has tweeted his support for President Trump's decision:

In this instance, calling on the National Guard appears to serve primarily as a show of force on behalf of the Trump White House that can be undertaken with greater freedoms and without the greater expense a full military commitment would incur.

President Bush spent $1.2bn on sending 6,000 guardsmen to the border to assist with the "War on Drugs" while President Obama sent 1,200 at a cost of $110m to the same end.

Historically, the National Guard has been called in in moments of national crisis, typically coming to the aid of civilians in the aftermath of natural disasters like last August's Texas flooding.

They have been assigned as a peacekeeping force before, however, notably in Little Rock, Arkansas, following the desegregation of schools in 1957 and during the Los Angeles Riots of 1992.