Entry by either side could be considered an act of war.
The UFP is banned from developing cloaking devices.
And actually, we only know that the Treaty of Algeron speaks of the latter
The Neutral Zone treaty might be a different beast altogether, and unrelated to Algeron. After all, such a treaty was in existence in TOS already, and supposedly was signed all the way back in the mid-22nd century, whereas Algeron has "kept the peace" and "tied the hands of Starfleet" for just sixty years.
Of course, it might be that Algeron originally didn't do a good job at keeping the peace, and only really got going a hundred and fifty years after its signing. And Starfleet might not have considered its hand tied by a fanciful "no invisibility devices" clause until Kirk's adventures revealed that invisibility actually existed and Romulans had it already.
But the intended idea seems rather to be that Algeron was only signed around 2310 (possibly as a reaction to the 2260s revelation that cloaks existed, and to a putative Starfleet countermove in the 2270s-90s, after which the two sides sat to discuss ways to avoid another war) whereas the Neutral Zone Treaty dates back to 2160 or thereabouts. Whether the no-cloaks treaty is an expanded version of the original NZ treaty, and what the proper name of the original NZ treaty might have been, we don't really know.