India has recently begun the process of turning one of the states of India into 2 territories with the Jammu and Kashmir Reorganisation Act, 2019. Disregarding the rights and wrongs of this, it seems to have been weirdly easy to have done. If this was attempted in the US would it be politically/legally possible? Are there any other federal countries where this could happen?
It would be much more difficult to do this to a US state, but maybe not impossible.
The US constitution does not have any provision for demoting or ejecting a state, nor does it (expressly) permit a state to leave voluntarily. Only the last, voluntary secession, has been attempted historically.
The closest subject addressed in the constitution is admitting new states and/or dividing an existing state.
New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
So a US state could be divided into two states, but it would require the consent of the state's legislature, not just an act of congress. Note also that this would turn the one state into two states, doubling the original state's power in the US Senate. It would not demote the state to a territory or bring it under direct Federal control.
Of course, the constitution could be amended, perhaps removing the requirement that the involved state consent. This would require ratification by legislatures (or conventions) in three-fourths of the states. This is a pretty lengthy process (and a high bar) but technically possible.
The equal representation of all states in the Senate is an "entrenched" clause, and not supposed to be subject to amendment.
no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.
There's disagreement about whether this entrenchment actually works, but it provides an argument that a state can't be "demoted" to a territory without its consent.
With the consent of a state, the federal government can purchase territory from the state. For example, Texas sold about half of what is now New Mexico to the federal government in order to eliminate the debt Texas had accrued before it joined the United States. Portions of what are now a few other states were included in the deal. The federal government included this land in various territories, before eventually admitting the final territories as states.
The Constitution expressly forbids denying any state two senators in U.S. Congress without the express permission of the state's government (in fact, it's so important, it's the only clause which can't be amended out under article 5 procedures). In theory, if a state, on its own initiative decides to not send Senators to Congress as well as representatives, it could be argued that it would be a du jure Territory of the United States, but I suspect that the actual recognition of this devolution of status would require support from Congress.
There are states that were once part of larger states that have been broken. A map of the 13 colonies compared to their modern size will reveal that many of them shrunk. There is a process for this in the Constitution, however, West Virginia famously did not follow this. It rather seceded from Virginia when Virginia seceded from the Union and Congress recognized West Virginia as the legitimate government of Virginia and the Confederate state as occupied territory (I prefer that we refer to West Virginia and Virginia by names that better reflect this: Virginia and Stupid Virginia, respectively, but I wasn't there when the south surrendered).
Perhaps the oddest example of this is the State that Never Was: Jefferson, which was to comprise of the northernmost area of California seceding from California proper and was actually scheduled for Congressional hearings... unfortunately, the day before there was a bit of a kerfuffle involving the Japanese over the then territory of Hawaii and the Jefferson hearing was put aside so that the matter of dealing with that problem could be addressed, and by the time it was all dealt with in 1945, support in California for Jefferson had waned.
There are actually a number of split-the-state style secessions with varying levels of support in many U.S. states today, and frequently, it's not so the parent state can have more power in congress. Most of the states with secessions movements will be a one party dominant state with the region supporting independent statehood normally being where the minority party holds most political power. Maryland's Panhandle is a good example as it encompasses a region of the state that is largely Republican in state politics but is gerrymandered into a Democratic dominant Congressional District. And of course, the Fragmenting of California, which even Californians agree might be best if broken up, but along which lines is not agreed upon. Along with California/Jefferson Split, There is a five way split (Including a Coastal state with L.A. and San Francisco keeping the state name, a Non-Coastal Southern California, a Non-Coastal Central California, A Norther Coastal-California (with Sacramento kept as the State Capital) and Jefferson (Basically the area north of the Northern-Coastal... which will also include a coast!), and a split of the interior California as the State of California and the Coastal independent nation of California (A variant splits the U.S. state portion so that the Southern region is cede to the Native American tribes in the area so that the Leftist who leave can still see the region not become a single Reliably Republican State). Contrary to its popular image, California has a very strong Republican base, and in fact from a period of 1952-1988, was one by the Republican candidate for President in all but one elections (It helps that most of these Elections had a Californian attached to the Republican Ticket. Nixon as Veep for 52 and 56, for President for 60, flipped democrat in 64, but back for Nixon in 68 and 72 (who wasn't voting for Nixon?), voted for Ford in 76 (Nixon was out, but He did Pardon Nixon!) and Ronald Regan in 80 and 84 (again, who didn't vote for him), and then Bush in 88 (Cause Regan couldn't run, but like Ford, he was "back-up Regan" and unlike Ford, Bush didn't pardon Nixon!). Since it joined the union, California has been something of a slow flip state, where it's reliable for one party for decades before siding with the other for an extended period of time. And If it seems unfair of me to mention how much California was reliable for Nixon, keep in mind, that prior to 52, Nixon was elected to the U.S. senate by Californians in 50 and the house of Representatives in 46 and 48. All I can conclude is that either Californians really loved them some Nixon, or really loved the idea of him going as far away from California as possible. Or loved both ideas at the same time. It is, after all, a state with a well history of Political Insanity.