When Laura Chinchilla won office in February, it seemed that the thorniest challenge facing Costa Rica's first-ever presidenta would be scraping together the funds for the crackdown on public security she had promised on the campaign trail. Two months later, the Costa Rican legislature voted to host up to 7,000 US marines for six months, solving Chinchilla's problem of delivering her law-and-order platform without emptying state coffers.

The controversial vote reauthorised the 1999 joint patrol between the US Coast Guard and Costa Rican police to fight narcotrafficking and provide so-called humanitarian support. Propped up by Chinchilla's centre-right PLN, the measure passed by a vote of 31-8 in spite of a walkout staged by six deputies attempting to break quorum and prevent the vote.

A week later the opposition PUSC party challenged the law's constitutionality in the Supreme Court. They argue that Costa Rica's constitution bans not just the establishment of a military, but also any occupation by foreign troops.

The US flotilla will include aircraft carriers, destroyers, fighter jets and nearly as many troops as Haiti received after the severest humanitarian crisis the region ever suffered (they will also stay in Costa Rica for longer). This year President Barack Obama signed agreements allowing US troops to occupy bases in Colombia and Panama.

The US government has not commented on Costa Rica's decision.

More perplexing than the motives behind US geopolitical posturing in the Caribbean is what Latin America's oldest, most proudly pacifist democracy hopes to gain from this partnership. Though Costa Rica remains one of the safest countries in Latin America, several recent high-profile cases of narcotrafficking have inflamed public fears that the drug violence plaguing its neighbours may prove contagious.

Last month, the Mexican authorities confirmed the identity of 14 suspects detained in Costa Rica as members of the Familia Michoacana, moving South American drugs through Costa Rica. Reports of local drug seizures fill the dailies and some residents complain of violence spreading in the underbelly of San José.

To an electorate worried about deteriorating law and order, Laura Chinchilla, Oscar Arias's vice-minister of public security from 1994, seemed like the perfect candidate. After her inauguration in May, she duly promised a crackdown on crime within her first 100 days in office, but the state's never-ending fiscal crisis ruled out investment in the inept police force.

Keen to remain a Latin American favourite of the IMF and international investors, Costa Rican politicians compete to outdo each other in their commitment to fiscal prudence, impairing the state's effectiveness. A recent World Bank report blames a lack of political consensus for the legislature's sluggishness in tackling rampant tax evasion, resulting in low tax revenue and "fiscal vulnerability". Lack of investment in infrastructure and social services threatens to undermine the country's development.

More than 60 days into Chinchilla's first 100, the Joint Patrol agreement seemed like the perfect escape from fiscal constraints. The wave of panic over drug violence that Chinchilla rode to victory two months ago has guaranteed widespread quiescence about the inflammatory decision.

"It's better to have US soldiers walking around the country than hitmen and drug traffickers," reasoned the anti-drugs commissioner Mauricio Boraschi.

The Joint Patrol gives the United States a disturbing space to pursue its geopolitical goals in Latin America. But the greater threat to Costa Rican sovereignty is a state so feeble and so preoccupied with fiscal discipline that it must continue to outsource its obligation to provide security for its citizens to its aggressive northern neighbour.

The danger of the Joint Patrol, like any drug problem, is that this political quick fix will develop into dependency.

Samantha Eyler Reid is a research associate for the North American Congress on Latin America and writes about Latin American and Hispanic American politics for nacla.org. She recently finished her MSc in comparative politics of Latin America at the London School of Economics.