  We consider the {\em MST-interdiction} problem: given a multigraph $G = (V,
E)$, edge weights $\{w_e\geq 0\}_{e \in E}$, interdiction costs $\{c_e\geq
0\}_{e \in E}$, and an interdiction budget $B\geq 0$, the goal is to remove a
set $R\subseteq E$ of edges of total interdiction cost at most $B$ so as to
maximize the $w$-weight of an MST of $G-R:=(V,E\setminus R)$.
  Our main result is a $4$-approximation algorithm for this problem. This
improves upon the previous-best $14$-approximation~\cite{Zenklusen15}. Notably,
our analysis is also significantly simpler and cleaner than the one
in~\cite{Zenklusen15}. Whereas~\cite{Zenklusen15} uses a greedy algorithm with
an involved analysis to extract a good interdiction set from an over-budget
set, we utilize a generalization of knapsack called the {\em tree knapsack
problem} that nicely captures the key combinatorial aspects of this "extraction
problem." We prove a simple, yet strong, LP-relative approximation bound for
tree knapsack, which leads to our improved guarantees for MST interdiction. Our
algorithm and analysis are nearly tight, as we show that one cannot achieve an
approximation ratio better than 3 relative to the upper bound used in our
analysis (and the one in~\cite{Zenklusen15}).
  Our guarantee for MST-interdiction yields an $8$-approximation for {\em
metric-TSP interdiction} (improving over the $28$-approximation
in~\cite{Zenklusen15}). We also show that the {\em maximum-spanning-tree
interdiction} problem is at least as hard to approximate as the minimization
version of densest-$k$-subgraph.
