The question raises two points:
- To what recent law does Thoreau allude?
- What is meant by "lack of sufficient force" to serve a writ of replevin?
The first point almost certainly refers to the 1843 Liberty Act, known popularly as "Latimer's Law". Latimer was a slave who escaped to Boston, but was there recaptured. Abolitionist meetings were held to campaign for his release, and eventually raised sufficient money to purchase Latimer's freedom from his owner. It created a massive upsurge in Abolitionist feeling - “The Great Massachusetts Petition” for Latimer's freedom collected over 65,000 signatures from Massachusetts voters - which resulted in Massachusetts enacting the Personal Liberty Law. Under this, Massachusetts authorities were banned from assisting in capturing fugitive slaves, and no state property (like jails) could be used to hold them.
The remark about the writ of replevin applies to the specific case of the escaped slave, Anthony Burns, whose arrest and trial in Boston is the subject of Thoreau's piece.
A writ of replevin is a writ authorizing the retaking of property by
its rightful owner. Its purpose here is was to have Burns released from prison by allowing him to demand a verdict on whether he was lawfully imprisoned or not - it was definitely not being applied to transport him back to his master! A full account of the process is given in Famous Trials by Professor Douglas O. Linder, but in summary:
The attempt to release Burns from duress by violence having failed,
steps were taken to accomplish the same object by legal process. For
this purpose resort was had to the Writ of Personal Replevin... The
first use of this instrument, for the relief of Burns, was made on the
day following his arrest. A writ of replevin was at that time made by
Seth Webb, Jr., and delivered to Coroner Charles Smith, who forthwith
served it upon the United States Marshal. The answer of the latter was
a quiet refusal to comply with the mandate of the writ, on the ground
that he held Burns by legal process.
Following this, various meetings were held with the Chief of Police and the Boston Board of Aldermen to see if a solution could be reached.
While this was passing, two citizens of Boston, Samuel E. Sewall and
Henry I. Bowditch, were moving in another direction and with a bolder
purpose... It was the desire of these gentlemen to have a writ of
replevin served with instant dispatch; they were quite prepared to
deliver Burns from duress without waiting for the Commissioner's
decision. But there was a serious difficulty in the way. Burns was in
the custody of an officer who had expressed a determination to resist
the state process, and who had a strong civil and military force to
back him. It was plain that if the writ was to be efficiently served,
if Burns was to be taken out of the Marshal's hands, it could only be done by the aid of a force sufficient to overcome that which he had at
Raising a posse was thought to be out of the question ("it was not to be expected that an undisciplined throng of civilians would be able to make head against the serried ranks and balls and bayonets of the Marshal's United States troops"), and the Governor refused to authorise the state militia to act in this matter. Accordingly, Burns was tried and returned to Virginia.