Washington is on the Potomac. It is accessible from the ocean. Troops from Massachusetts, or from Pennsylvania could have been brought in by ship. The reason to parade them through Baltimore was as a show of force which was NOT well received.
From Wiki:
A man supposed to be a Maryland State Militia soldier was detained in Ft. McHenry, and Judge Giles, in Baltimore, issued a writ of habeas corpus, but Major W. W. Morris, commander of the fort, wrote back, "At the date of issuing your writ, and for two weeks previous, the city which you live, and where your court has been held, was entirely under the control of revolutionary authorities. Within that period United States soldiers, while committing no offense, had been perfidiously attacked and inhumanly murdered in your streets; no punishment had been awarded, and, I believe, no arrests had been made for these crimes; supplies of provisions intended for this garrison has been stopped; the intention to capture this fort had been boldly proclaimed; your most public thoroughfares were daily patrolled by large numbers of troops, armed and clothed, at least in part, with articles stolen from the United States; and the Federal flag, while waving over the Federal offices, was cut down by some person wearing the uniform of a Maryland soldier. To add the foregoing, an assembly elected in defiance of law, but claiming to be the legislative body of your State, and so recognized by the Executive of Maryland, was debating the Federal compact. If all this be not rebellion, I know not what to call it. I certainly regard it as sufficient legal cause for suspending the privilege of the writ of habeas corpus."To my mind, a darn good argument. I've noticed that Democrats have a strong tendency to bend/break the law without breaking stride when attacking their enemies, then complacently assume all the protections of the law will be available to them.
The problem is that once you "appeal to arms," as the southerners were so fond of proclaiming before and in the early days of the war, sometimes the appeal goes against you.
Then they suddenly developed a fondness for legal and constitutional methods.